is it true brushless motors only work with lithium batterys?

Not even close to true

Brushless motors will work with both li-po's or regular cell packs. You will get more power with a li-po pack as the voltage is higher. Also li-po batterys hold there voltage longer than ni-mh packs and usualy have a higher amp hour rating . The are also smaller and lighter considering their power output and capacity. Using li-po batterys with a brushed motor will give you incresed performence and longer run times too.

What kind of run time could one expect from say a six cell Gp1100 pack?
Lookin at the Mamba comp or the new Tekin Mini Rage (6.8kv or 8.0kv).
Brushless is totally alien to me just starting the research. The system will be installed into either a Mini-T or an M-18.

Thanks, Voodoo...

Figure on atleast 20 minutes, I can get that with my RC18T with a brushed motor and GP1100's. I actually get tired of running before I dump, brushless will only get you more runtime.

I would say 15-20 minutes, but it will really depend on the motor you chose. I could get over 30 minutes with a Mamba 5400 and 1900mah lipoly pack.