Hi mungodaisy
Clever hedgehog! But no, they aren’t really house-trainable! I have heard of them getting through cat flaps before – probably easier if it’s one positioned quite low down, but they are also better climbers than many people think. Occasionally they are found to have been upstairs in houses!
Yes, cats and hogs normally tolerate each other. Although a cat might not try to eat from a bowl already occupied by a hog, but it depends a bit on the respective characters of the hogs and cats.
If it’s a problem, the only solution I can think of at the moment is those cat flaps that only allow cats with collars which have the right ‘signal’ in them to pass through them.