That sounds interesting!
I think it should be fine citing Housetrails as a source and vice versa trying to present and link buildings in OHM.
Concerning the licence I just took the one from Mediawiki projects since I don’t have much experience with that and thought that this will be ok. Allthough I prefer a free licence and try to get as many pictures as possible under this licence, the pictures also allow more restriced licences than the Wikipedia for example.