Red devils didn't get that name for nothing - they can be very aggressive. The easiest way to sex them and a lot of cichlids is to look at the back ends of the soft dorsal and anal fins - if they end in long filaments, the fish is prolly male, if the points are kind of short - probably a female. The pic of your fish is really tiny, but it looks like a girl to me. Males can also get a LOT bigger, and sometimes develop a hump on their forehead.
I wouldn't put a breeding pair of red devils in anything smaller than a 75, but a 125 would be a lot better. In either case I'd have a divider made of plastic egg crate diffuser grating pre-cut and ready to put in in a moment's notice.
Red devils can have tank mates, but they should generally be other aggressive cichlids. Even then individuals vary, so you never know. It's really hard to put another fish in with a fish like a red devil if the RD has lived alone in the tank for a while - If I was adding another fish or two I would move them all at once into something like a 125 so it's new to all of them. In a tank of your size, I would be content to keep it as a specimen tank.