This is practical problem which came during conversation with other players. A few small players would like to unite against common enemy.

Can a fleet be composed from different player’s ships?

If such fleet successfully attacks a colony than the colony should belong to someone after the attack.
There are many possible options here. The colony can belong to:

a) The strongest player in the fleet
b) The one who has special admiral ship (outpost ship will do)
c) A designated player agreed before the attack
d) Belong to all attacking players