The breeders allow their unneutered Bengals free rein?
These don't sound like registered breeders to me.
Bengal studs can cost a couple of thousand pounds and queens can cost between a thousand and two thousand pounds. It doesn't makes sense that someone would pay that sort of money then allow the stud out to contract FIV, or fertilise the local moggy with his pedigree sperm. Same with the queen - why would the queen be allowed to wander free to be impregnated by other boys, possibly carrying diseases?
Anyway.. I don't see why there would be a problem introducing another cat, as long as the new cat has a similar personality. For example if you get a sleepy Persian that Persian is going to get most irate at your Bengal's ADHD personality and the problem will be made worse, not better.