I voted I think it's okay for some pets. You know your fosters and their personalities. You know who could handle it and who cant. So just use your judgement and do what you know is right for that specific kitty.
It sounds like Zach might be able to handle it, but it sounds like Calloway would NOT. But if you don't want to put Zach through it, then don't.

And thank you for reposting my favorite picture of Zach (the first one).