Are bell transformers allowed to be installed in a panel with circuit breakers, or fuses as long as there is enough gutter space for one?

Not that I have ever heard of. This would mean that the class 2 wiring from the output would be occupying the same enclosure. That's not to say that I haven't seen that done more times than I care to count.

Kind of like 10-2 W/G serving a dryer, it,s always been against the code but, seen it done way too many times.