[Allfather]

No problem having private galaxies.
A galaxy could be 3 or 4 planets, should be ok.
A galaxy can join an "interstellar group" with up to 3 other galaxies.
These galaxies would have an eta advantage of -1 or -2 on defense
They could split apart, but only every 200 ticks or so, this would reduce people "jumping ships" to often.

With p2p selfexile should be impossible for private galaxies & randome galaxies could have max 1-2 exiles per round (per member).

If the galaxy is inactive (certain % of them) they will get mixed with other galaxies in the same predicament, and the inactive could be exiled into c200 and stay there until activity increase .