Yep, TOS BSG. I started the campaign several yahrens before the holocaust, so that the players could jump around between the Colonies. I started introducing some NuBSG concepts, making it a hybrid world, including humanoid Cylons and new-type "experimental" Battlestar designs with new kinds of propulsion systems (jump drives), with the TOS mythology still at its core. I moved them through the holocaust, on a parallel path to Earth as the Galactica and Pegasus, and then at some point, it evolved into a BSG/TNG-era Trek crossover, in a three-way final battle between the Seraphs, Iblis and Q, with the Colonials, Cylons and Alpha/Beta Quadrant species serving as their unwilling pawns, with a Borg/Cylon assimilation encounter that made things even more interesting. I also threw in a lost tribe of cybernetically-enhanced OG Lizard Cylons (flying Ralph McQuarrie concept Basestars and Raiders), who allied with the Colonials to help destroy their robot creations once and for all.
I based a number of my later crossover storylines on the unfinished "
Dark Dawn" fanfic trilogy by BJ Henry. It's a great series of stories that was, sadly, never completed - only two of the three exist. I found a lot of inspiration there from what he had written, though.
Needless to say, the group had a blast. Campaign lasted around 3-4 years.
I didn't really come up with any specific rules, per se. It followed the same D10 die roll system of the original, but I'll see if I can dig up some old character sheets.