As I'm not yet experienced enough to speak intelligently on questions 1 and 3, I'm only going to adress question 2.
 
The only race I'd like to see added to the game are Tyranids.
 - I'm make their game pieces purple.
 - I'd give them a massive pool of tier 0 troops, with plenty of ways to get them into play fast and cheap.
 - I'd give them a very limited pool of tiers 1+.
 - I'd have combat cards key on higher-tier troops to grant bonuses to tier 0 troops to reflect synposis.
 - I'd give them ways to easily generate tons and tons of reinforcement tokens.
 - With so many ways to generate cheap or free tier 0 troops and reinforcment tokens, I'd consider having them be automatically destroyed if routed.
 - I'd consider giving them an alterante win condition (such as take over any four planets containing an opponet's buildings) if it could at all be balanced.

My experience is...well...the opposite. 4 player games seem to be balanced. 3 player games not so much. 2 on 1 action has a nice ring to it, but is horrible in a game like this.

Actually, I found the game to work rather well with 3 : Getting your objectives being more important than denying the others, you have little incentive to expose yourself too much in doing so.
I found it to work much better than Eclipse, Twilight Imperium, or even Nexus Ops.
But it is true that hexagonal tiles and a stup like TI or Nexus Ops would have worked better with 3 players . Starcraft with Brood War scenarios might work better with 3, but there are not that many combat games I can think of that do.