I can appreciate the rescue. Imo odds are he wouldn't fare well with the bigger, healthier JDs in the other tank, I expect they already view it as their tank if they've been in there a while, especially if they've formed any kind of pair bond, and in a 45 there's not a lot of territory to go around. With JDs and a number of other cichlids it's not only about enough gallons, filtration, or water changes to keep them healthy, it's about real estate (tank dimensions), especially once they've put on some size. If the ones in the 45 are 3,4 inches or near it, they're potentially not far from trying to breed if they're a pair. Not a scenario that will make them welcome strangers.
My thought is I'd plan on fish in both tanks needing to graduate to bigger tanks and for starters I'd try and move the 45 gal pair to a bigger tank and the 10 gal fish up into the 45 before long.
