You can run the guest house from the main house but the only way to know for sure if the 200 amp service is large enough is to do a load calculation. Are the appliances, HWH, and heat gas?
I would agree with Robert, as long as gas is doing most of the heavy lifting and the grounding and bonding at the various panels is correct I would say it is fine. If no gas is present I would then strongly recommend a licensed electrician do a load calculation.