Got one other idea...two mini mains...seen it done before...lower mains are 7-10 mins, one race. A main is two 7-10 min "rounds" where you race for 7-10 mins. The race ends. All trucks come off the track, swap batteries and another 7-10 min race starts. The results of the two races are added together. Usually it's done so there is a break, like this:

Truggy A1
Buggy A1
MT A1
Truggy A2
Buggy A2
MT A2

It gives everyone a chance to compete under power.

But personally I'd rather just see ALL mains be 10 mins. Keep it simple and keep it fair. You don't want guys not to show up because they know they can't finish...