I don't know if this was covered, but I don't think it would be necessary.
Because the answer is obvious.

The scroll most likely wasn't quite same because Kakashi had to use it in order to make the summoning. Jirayia doesn't.

Of course the contract is to one animal. And kakashis just happens to be with dogs. And I think u can make it with almost any animals, but I don't know or try to guess that can you make more than one contract at same time.