I'm still pretty new so I've been trying to work this out myself lately as well, and I've come to some conclusions. If you're trying to use sensors optimally on ships in your main fleet, I think you should ideally have a dedicated ship type for each type of sensor you want to use - both passive types and one for each resolution of active sensor you think is relevant. Sure, you can put small backup sensors on large ships for just in case they happen to get separated so they can fire their weapons, and you might want to put a lot of redundant small active sensors on jump point assault ships, but small sensors don't actually do anything for you if they're always in the same location as a larger one of the same type.
Ships in an efficiently built fleet will be limited to certain maximum tonnages because of jump drives and shipyard capacity and various other considerations. Because of that, I think instead of having two equal-sized large sensors on two ships, it's almost always going to be smarter to have one sensor that's twice as big on one ship and a different type of sensor that's twice as big on another, or more defensive capability instead of the second sensor. And then if you want more redundancy, build more ships. I've seen a lot of ship designs with both kinds of passive sensor in equal amounts on them and if you're doing that, think more about the ship's role, what it's searching for, what it needs to defend against, and whether it wants more thermal range or EM range.
In addition to the big primary sensors in the fleet itself, to extend the sensor capabilities of your fleet, you should have pickets in formation around it at set distances and angles. With pickets, IMO the main thing you're trying to do is to keep your fleet from getting painted by an active sensor, so EM is maybe the most important capability for them, you want to try to spot small enemy scout ships with high-R sensors before they can get into range. You'll also probably still be covered by the high-R sensors of the fleet, and high-R sensors have such high EM emissions that they'll make your pickets easy to find, so they should have low-R actives. Thermals I think are less important on defense - the only threats to your fleet that 1. don't radiate a lot of EM 2. have a small enough ship size to avoid high-R sensors and 3. also would show up on thermals would be fast small ships with short-ranged weapons, and detecting those very far out from your main fleet doesn't do too much for you since they can't do damage without closing the distance anyway. Speaking of fast squadrons of small ships, those are probably what you should have to quickly react to any enemy scouts your pickets find.
Out beyond your pickets I think there are two more classes of scouts you'll need to have - scouts with active sensors and stealth scouts with pure passive sensors like liveware's SS-2 up there. Because missile fire controls have 3x the range of active sensors, you need forward scouts to target enemy fleets so you can launch missiles from your main fleet at their maximum range, without giving the enemy fleet the opportunity to respond in kind. The types of sensors that are useful on scouts are conveniently the opposite of the ones you want on pickets - you need long-range high-R active sensors (that should probably stay turned off most of the time since they will give your position away to EM sensors) to target large enemy ships for your fleet. And you need thermals to passively find good targets that may not be radiating lots of EM - commercial vessels, for example, or pure beam warships. For scout ships with active sensors to survive, they need to only turn their actives on long enough for the missile engagement to complete, then they need to shut their sensors off and reposition at high speed. They'd probably benefit a lot from cloaking. You can also use long-distance missiles with active sensor buoy second stages fired at a waypoint to light up enemy fleets that you have on passives. Scouts with only passive sensors, though, can do their job without ever necessarily giving away their positions, and so they can be extremely stealthy and small and get right up close to the enemy. You can also use passive sensor buoys that can be dropped by a minelayer or fired at a waypoint as the second stage of a long-range missile. These are the things your pickets with R1 active sensors need to sweep for, they are almost impossible to spot any other way.
Then you have other classes of ships for which sensors are important in your rear areas, where hostile aliens may arrive through dormant jump points even if all your borders are guarded. You've got to protect your colonies, jump points, and commercial ships in similar ways to a fleet. Your DSTSs are very strong EM/thermal sensors but like all sensors they run into diminishing returns, so you probably want to put networks of sensor buoys all around your systems like you have pickets around a fleet, to cover the approaches to your colonies all the way through their orbits. The same sensor type considerations apply here as well - for defensive purposes, you want EM buoys rather than thermals to detect long range active sensors and missile fire controls. Thermals are more for finding targets if you have, for example, huge missile platforms orbiting your colonies. Instead of/in addition to buoys, you can also have long-endurance patrol craft with looped orders to repeat routes around outer systems and then overhaul at your colony. As for active sensors, your STOs have small R1 actives so they can fire, but you need to put defense stations in orbit around your colonies for them to have high-powered actives. Pound for pound, stations are stronger than ships, and colonies can base fighters in relative safety on the surface, so you may want to use a combination of mostly stations and fighters rather than ships to defend systems.
The difference between sensor protection for military fleets and commercial ships is that you can try to avoid getting picked up on passives if you're escorting a commercial convoy through a risky area - commercial ships are limited to low active sensor strength and engine power so the thing that'll detect them at long range is very high-R active sensors, which are clear as day on EM. So good EM sensors are IMO the most important sensor type for commercial convoy escorts, and you probably want to have an advance scout travel the convoy route ahead of them to look for danger and drop buoys. Truly massive commercial ships, stations, and tugs might show up on thermals from across a system no matter what you do, though, so you might need to give them a full combat fleet escort if you're towing a multi-million ton habitat station through a combat zone like a madman.
Then with jump point assault and defense the main thing you need with sensors is just redundancy, everybody will be coming through within the range of small R1 sensors but your ships are going to get shot at a lot, you don't want an expensive HS50 sensor anywhere in the area, passives won't do a lot for you, you just need to not have literally all of your active sensors shot out on all your ships. The same thing applies to colony invasions, almost any R1 sensor can see the entire battle area but lots of beams are going to be going back and forth.