In my experience I have never, ever been able to keep flower horns in community tanks. I truly believe they should be kept alone. I believe they're one of the few exceptions to "every fish is different"
As of right now the male Sal can keep up with the FH. Granted the FH is still young so going to keep an eye on it. For now it works but who knows as the FH matures. Or even if the sals are a pair. I have heard the sals as a pair can be super agressive.