I'm new to the game, only had it a few weeks, but I'd think more races would make the game easier in many ways. If you put anything into diplomacy, or play one of the mind control races, you can have most of them attacking each other the entire game, while you sit back and build. You'd also have a lot more customers for trading or selling techs and trade goods. Plus, if you go heavy on influence, who needs transports? Just assimilate the planets that used to belong to the competition.
Of course it also means more races to declare war on you but, since I stopped playing the Terrans, I don't think I've gone more than 40 turns without someone deciding to attack me, no matter what my military is like. I think it'd also be easier having four smaller races all at war with me at once rather than one or two large races with good economies and production from a large number of planets controlled.
A couple of games ago I played Huge/Four, and one race not only borged most of the space in record time (I think they must have started the game with a fleet of hyper warp 3 colony ships and constructors), but started a war with me that never ended no matter how many ships I destroyed or planets I invaded....
...hmmm. I think I may have just talked myself into using maximum opponents too.
