Sorry for the late reply, hopefully you've had a game or two on Tough by now...

I wouldn't say it's too hard at all - I tend to play on Masochistic or Painful (depending on whether I'm using a race I'm good with or new to)and the main problem is dealing with the AIs' starting advantage - they seem to research early techs and build early achievements really quickly as well as putting colony ships out ridiculously fast, but then it gradually evens out.

If I remember correctly the AI gets smarter from Normal to Tough (which is actually very satisfying, as you start seeing the other races' play become a bit more human-like) but after that it's all in the bonuses and you just find the level which best balances challenge and enjoyment.