My cats took about 6 mos to stop hissing at my dogs and new kitten when each was brought in. I say as long as no one is in danger, they'll find a way to live together. Might not be best friends, but they should figure out a way to coexist. Sometimes the kitties take a while to realise the new one isn't going anywhere and they need to just get over it, which they do.

I think it's great to adopt an older dog in need. Even a younger dog may not be with you for too long, so just enjoy the time you have with them!