considering another rescue

Just because the Queen Bee isn't non-stop gung ho about other dogs doesn't mean she wouldn't do fine with another one in the house. From your description of how she acts at dog parks it sounds as if she doesn't mind being around other dogs at all, but that she's also perfectly fine entertaining herself. My guess is that as long as the new guy is the type who respects other dogs' space she'd probably do fine. That doesn't mean there wouldn't be an adjustment period.

Much more often than not I've seen "only" older dogs benefit from bringing in a companion.