The guidelines for integrating any new cats into an existing household of cats are always, "slow and easy." Maybe if you were to slowly introduce the "newbies" to one or two of the other cats, in their upstairs area where the fosters feel safer? If those cats got along well, you could introduce another of your own cats, then another (I'd suggest saving your two territorial cats for last), until you could finally allow the fosters access to the whole house.
On the other hand, you mentioned that the foster situation is only for four months, while the owners are travelling. This will untimately play a role in your final decision. Which will be less traumatic on your own cats? Being confined to the bottom half of your home for four months, as you stated they presently are, or making new friends, only to find them disappearing in a short while?
If you do decide to integrate, it will be a process of trial and error, and with eight indoor cats and the two fosters, you certainly will know very quickly what works and what doesn't.
