You can definitely do cross on a mountain bike and I think riding on off-road will improve your cross skills. I wouldn't bother trying to get a used cross frame...there aren't any that will fit us!

I had sent an inquiry to Georgena as well and got the same reply. I'm definitely going to be checking them out tomorrow. She said they are building them to order and it would probably take about 6 weeks to get one. Plenty of time to have one by next fall!