To shuttle minerals from one site to another, like, from mars to earth, you need a massdriver on both sides, one on mars and one on earth.
As you currently likely have a good amount of pop on earth, I would build manned mines and ship existing Automints to Mars, Mercury, and Venus, whereever you have a good amount of minerals.
Also, start building Infrastructure on Earth and ship it to Mars.
Once you have a sufficient amount to support a million or so, start transferring colonists. (If you can produce and ship Infrastructure the same speed as you can bring colonists, you can start right now, though)
As far as I know, you need a new fleet for every own task, so no, you can'tmake parts of a fleet do different things.
You can activate active sensors either at some targetable place, with an Order, for Example, "Mars, Active Sensors ON", or you can do that in the ship description or maybe combat settings, can't quite remember.
Parent fleets are mainly used for combat as far as I recall.
It's adviseable to build Jumpgates
Also, you can use Jump ships as jump tenders, place one on the point and use 'Standard Transit'.