The ICBMs that are available at the start of the game are of size 24 thats why I want to use that size, I was thinking of maybe doing a multiple warhead version of it however were there are maybe 3-9 second stage missiles instead would that be a better alternative?
Well apart from role playing there is really little reason for size-24. But role playing is valid in my book, so why not size 24. The issue from a game perspective is that one size-24 missile is just as easy to intercept as one size-4 missile. So if you launch six size-4 missiles you are much more likely to overcome enemy anti-missile defences. But to actually answer your question: Yes, separate second-stage missiles would be a good move, just make sure they separate from the main missile at sufficient distance from the target (so that the main missile wont get shot down before releasing its submunition).
Yes I was under the assumption that hit chances were extremely important.
Why is the speed of 10 k km/s important, do I have too fast missiles or is it something else?
Hitchances are just one aspect. Think about it in terms of “expected damage”, i.e. the product of hitchance and the warhead. You are very much likely better off by emphasising the warhead more.
Your missile is 107% sure to hit a target moving 10k km/s. Of course hitchances are gets capped at 100%, so you waste a bit of missile space already. The argument becomes even more critical when applied to more likely target. Say you encounter an enemy moving 5k km/s, then you would have a hitchance of 214% - way over the top!
Oh, thats nice, I'm guessing that the sensor might not need to be a combatant station either. But does one need any passive sensors on PDC ever?
Correct. I typically have one “command PDC” with heavy armour to host the active sensor, and several outlets that are nothing more than launchers and firecontrolls, small enough to be pre-fabricated in one section. There are so many of these outlets that an attack on these should be rather frustrating, forcing an attack to commence on the much more heavily armoured command centre.
You do not
need passive sensors ever, and if you have ground-based tracking stations then they are completely superfluous on PDCs.
That's also very nice, I'm guessing that that reload will be automatically as fast or faster than any other reload, is that a correct assumption?
Refilling a ships or PDC’s magazines from a population is instantaneous (you can do so between turns), the subsequent reload of the missile tubes occurs at the normal rate, yes.
The range is because of me misreading numbers 
, the warhead will be fixed in the revision, I just have no real idea of what scale damage comes in in an ordinary fight.
Will the sensors target other ships automatically if necessary and why would one use thermal and not EM or Active sensors? Agility will also be fixed in upcoming revision.
It will automatically target enemy ships that are in range of the sensor, if its primary target has disappeared. Active sensors emit radiation that is detectable by enemy EM sensors, which has the effect of providing an advance warning of an impeding missile attack to the enemy. If he knew that already it wont make much of a difference, but often it is possible to get in one salvo before he knows what is happening if the missiles do not have active sensors. EM sensors only home in on enemy active sensors and not all ships have theirs activated, while all ships do emit thermal radiation.
If you post your tech specifics I could run a little simulations to determine a likely best missile. I would need your engine, warhead, agility and fuel efficiency techs, along with your enemies observed best speed.
