Well, I tend to agree with Ace in most parts. 100's of ships.. how many polygons per ship? each ship having at LEAST one texture, that means 100 textures +

of polygons.. that is, assuming you were to make it 3D.
Using the Tribes Engine... is that copywrited?
Using other's mods... is that violating copywrite laws as well? or do you just plan those for the unofficial modded parts.
And as far as space/planet combat, are you going to transcent from space TO the planet, or just have it simply switch to the ground scene when you reach an objective.
This game does sound too ambitious. But, if you do somehow manage to pull it off, I'm for ya all the way. However, remember the term "Mech" is copywrited by Fasa.

As far as the cliche's go, Well, that's up to you. Personally, I haven't seen anything "new" in the Sci-fi world in a long time.
Now for the suggestions:
Aliens... First, don't make them all "carbon based"... What if some are gaseous in nature? some aquatic...ect. Most of the alien ideas have already been done in one form or another, so, making it unique will be impossible.. somehow try to make it original. OR, do what Ace suggested, make it all human based fighting each other.
Build a resource structure to limit the ammount of ships. Sure, it may lead to "rushes", but two types of games seem to make it big... Either combat type with one pilot only, or strategy resource dependent games. The only game I know of that combined both fairly well, was Battlezone.
How long is the typical game time going to be? If it's too massive and too long, nobody can finish a game online. Trust me, I've seen a few online games that take too long to finish, they never become popular online. Somehow, you'd have to limit the game time to 1 hour. Any longer will be too much.
Also, limiting the bandwidth for online gaming rules out

of people. Remember, not everyone as broadband or can get it. Making it broadband only pretty much rules out all places except the US. That's

of income potential loss there. Also, if a 14 year old child wants it, his momma must be pretty rich to give it to him to play online games.

Space engine, like ace said, should be semi-realistic. Personally, I like the I-war engine the best from ALL space engines I've played so far. That'd be my suggestion.
I don't know if it could be possible, but item upgradability would be nice. for example, thickening the armor of a mecha or changing the weapons, but you'd have to have weight/energy/speed ratios. thicker armor would mean heavier/slower.
And last suggestion... don't make any "super weapons". In all my gaming experience, the only units used online, are the most powerfull. All the good games were based on an even weapon system. the Mechwarrior system had heat to balance out the power of the weapons... Starcraft had resources to balance it out, ect.. But, if there were a Single most powerfull weapon (like LRM's in MechCommander), then you'll only see those used, and not any sort of medium in between.
Just my thoughts.
Neal