Awesome, I Can help if ya need, Ive modded in a few games before, dont know much about modding this one though. But I loved the ship battle in serenity. Never seen firefly though, did they have any awesome ship battles in it? But ya ive always wished someone would make a mod based off of serenity.
XFIRE=Dooms1988

Would have suggested this myself but I kinda have my own mod...
And the races could be Reavers vs. Feds. It’s in the end of the serenity movie just make most of it up

boo silent space, it makes for great dramatic moments in Firefly and BSG, but not so much for games

If you need canon research for this game, I offer my services, I own the series, DVD, and Those Left Behind graphic novel.

Ya might get everyone playing as the Reaver race....  

I think Firefly was one of the greatest shows ever, and Serenity was a kick@ss movie in general... That said, I have to agree that there just isn't enough material to work with to mod a game as huge as Sins. That show was really more about the crew than the universe they were in, and it was a crew of miscreant freelance pirates rather than combat pilots. Add to that that it was less than a full season before it was cancelled. Now a Firefly mod of something like Sid Meier's Pirates might work, or Freelancer, but an RTS game is going to be difficult. I don't want to be a complete pessimist though cause I'd LOVE to see you pull this off! So here's my suggestions.

To put the Firefly universe (not the crew/ship from the show) into an RTS would be best accomplished by what MachinaManiac said. Make it a prequal to the TV series involving the war between the Feds and the Browncoats. Then I say use the Reavers to replace the Sins pirates. Utilize the culture and trade features more while using direct fire combat less. Especally since we have no idea what ships the Browncoats had, but they were presumably less in number and power than the Feds. As for the empty space feel of the Firefly universe, it was less "emtpy" while the war was in full swing, but you could downsize the ship counts and planet strat/tact slot counts to help it feel lighter. Also, don't feel like you have to fill all the ship types/roles in Sins. Just fill what you can with out stretching the "cannon" too much. Lasty, for this to be true to the series, the Browncoats should be the underdogs anyway. So don't worry about the usual RTS need for balancing either. Just make it more of a roleplay challenge to play Browncoats rather than a balanced chance to win. Most of your audience for this mod are going to be fans of the show anyway, and I think they'll appreciate your accuracy to cannon more than stretching it way out just to fit the Sins 4x/RTS model.

Best of luck! I'm looking forward to seeing what you come up with!