I was thinking of getting a flowerhorn for a while and for 1 fish you do want at least 55gal. One fish can live comfortably in 55gal it's whole life.
In terms of tank mates.. most likely you won't have ANY. That's partly why I didn't get one. If you have a big tank that you can stock heavily with lots of hiding spots, you have a better chance of low aggression, but often you can't even keep a male and female together due to aggression issues. Though I was told if you wanted to attempt having multiple fish, get them both as babies or juveniles and they will be more likely to get along as they age.. but that's not a guarantee. Always have a divider on hand JUST IN CASE.