I'm currently in my second game ever on Tough difficulty (got the game a week ago). Similar to HawkUnit, my playstyle focuses on heavy research and economy. I found the Altarians do be optimal for me.
During my first Tough game I had the same problem as HawkUnit: I started researching weapons and such way too late. Then the Korath declared war on me (Sporeships, oh I hate them). I could stall them and eventually got a peave treaty with only minimal losses.
However two turns later the Yor attacked me, and they had the strongest military of all races in that game. The Altarain racial bonus didn't help much, all too far away. So I surrendered.
Fresh start, same race, same playstyle. Only this time I forced myself to alternatively research combat and non-combat techs. I also got trade early and sent freighters to my closest neighbors. Later when I noticed my neighbors started to militarize, I already had medium hulls, some Laser IV and some miniturization. With my strong economy I then bought several ships and positioned them in strategic locations.
The only race that has declared war on me so far were the Thalans, but only because I wanted them to do so. An influence starbase in the heart of their territory works wonders
Fast forward a few turns: 500BC plus each turn, best reasearch and economy of all races, thalans succumbed and an average military rating. That's where I am now, I predict to win this game

To summarize: Like others said as well, it's important to research military techs early and to try to keep up with the military strength of the other races.