I do believe that Rowling had placed Teddy in Hufflepuff like his mother, I just can't remember the source.

Also, there are probably 7 rooms in Grimauld, since I don't believe that the Weasely parents moved between houses in book 5. So, one for the parents, one for Sirius, one for the Twins, one for the girls, one for the boys, Regulus's old room was left alone I think, and there might be one more room for guests. It is likely that the Blacks had a fair number of guest rooms, being wealthy and all that.