Sorry to bother you with this. So right now, max of 6 mercs can be in one squad. Perfectly fine. I now have two squads, one with 3 and one with 4 mercs. Unfortunately, the game won't let me have both squads in one sector. Is this on purpose or just not implemented atm? Can't attack an occupied sector with two squads, which is sad :(

Also, moving one merc to another squad took me forever to find out how ;)

I think you can defend with two squads if they're in the same sector when attacked, but not attack with two. 

Moving mercs should be improved once the new strategic UI is in.