Eventually TL will outstrip mechs as they hit the equivalent of a technological dead-end. It would possibly be feasible for a few TLs, say 9-11. Grav tech would quickly outstrip it as development on it progresses - possibly be unreliable for about one TL while all the bugs are worked out. Maybe 2 if you push it.
Mechs, by their very nature (robotic) would be fragile. Combat would definitely take its toll quickly, and maintenance costs on them would be (I expect) quite high. If mechs are developed, they would likely be modelled after those in the manga/anime "Appleseed", or after the Humpty-Dumpties from the RPG "Battlelords of the 25th Century" (I *think* it was 25th, anyway).
Rocket/jet assisted mechs could move quite quickly, but stopping may be an issue (imagine what the landing loads would be like, not only on all the mech's joints, but also the loads on whatever surface they landed on). Gundam-style (actually, any giant-mecha-styled) combat moves would be pretty hard on both the machine and the environment. Although giant mechs might be fun to have, a hard look at the science behind them would quickly show that cost-effectiveness and robustness is more easily found in more traditional ground and air vehicles.
IMTU, I have Battle Movers, which are upscaled BD able to hot-swap weapon loads. However, that's the ONLY real advantage they have. They're hard to maintain, not very fast (around 30-35 mph continuous, 15-20 mph in adverse terrain), and lightly armored. If they every fall, they're pretty much toast - a lot of the motor tech in the BMs is geared towards keeping it from falling. They may be flexible, but one good shot from a throwaway rocket launcher, and it's hasta...
If you really want Gundam/Nadisco styled mechs in your game, try Mekton, Heavy Gear, or some other system where it can be pulled off without having to stretch the science too far. IMO, I think you're trying too hard to justify them in a technically "hard" SF setting.