Right, so I tested out an expeditionary invasion force:
Fleet with 1 Transport (500-1000 soldiers on board), some frigates, a destroyer.
* Fleet system works pretty well. Need a single Fleet disband button, I think.
Captured a pair of planets this morning.
After invasion, you get to keep the Transport! That's kind of nice. You still have to build a bunch of new soldiers to fill it, but that's a big outlay of expenditure I'm glad I don't have to buy again.
One planet, I kept all of the buildings and population. But the second lost a chunk of population and most of the buildings were destroyed. I'm guessing that's due to a difference in player race/planet population max.
Otherwise seems to work well.