Hey Doopa! I'm an aerobics instructor and I'm not 400 lbs...but I've seen several that are pretty large...yep.
I think you should be an instructor..go to
www.afaa.com and read up on it...or go to
www.acefitness.org...they issue the certifications. You'd be great.
and...don't give up just because you have no partner...look for someone at the gym you can partner up with...someone not too lame....maybe someone who is thin and has pie filling on her face..
similar fitness goals, you know.