For an adult male flowerhorn, a 75 gallon is minimum. We're talking about a potentially 12" or larger, and very chunky fish. For a female, a 55 gallon might do, depending on how big she eventually gets.
As for keeping your two FH's together long term .... forget it. It just won't happen. FH's are incredibly intolerant of any other fish in their territory, once adulthood kicks in. To breed them, on must only allow them temporary access to each other. My advise is to sell off the one you like the least, and buy a 75 gallon for the other. Unless you plan on trying to breed them, in which case you will need two big tanks, or one very large tank (something like a 120G) with a divider. Keeping adult FH's in tanks with other fish can be done but it requires a huge tank (something like 200 gallons plus).
As for their lack of interest in pellets, just give them time and persevere. Keep trying. At least they are showing some interest and mouthing them. You may need to starve them for a few days before they will eat the pellets. Also might be a good idea to try a different type of pellet, as often fish may completely refuse one type, yet eat another. A diet based on bloodworm is not good at all for omnivorous cichlids like FH's. They ideally also need some veg matter in their diet, so some veg-type pellets would be good in the long run, once you have got them more interested in pellets in general.
Good luck !!! Some pics would be nice.