I mirror what Suzi said - even if he has been neutered it takes a while for his sperm to die off and if there are any traces in his tubes there could be some unwanted pups. Also, it takes a good while for the testosterone to leave his system so his urges would still be there when he came home from the vet and he would probably be up on her quite a bit.

I think it is great that you are getting him neutered and I also think that it would be a good idea to get her spayed in the future (provided she is big enough to get it done, some of the "mini" breeds cannot be done because they are too small).

My advice would be to seperate them (major pain in the bum), perhaps let him stay at someone elses house after he has had his opperation but while she is still in heat. It would be the easiest solution that I can think of because trying to keep them seperated in the same house is a nightmare.

Good luck!