Each fish should have 2-3 shells so they have a choice.
I actually am in the same situation as you,
For brevis the males will grow alot larger than the females abour 3 inches Vs 1.5.
One of my smaller fish turned out to be a male, hes currently separated because the two big males were beating him up.
I wouldn't worry about the female getting beaten up, because they should pair up (when they do the male and female might share a shell, or live in shells next to each other, usually something like that). Once they pair up the male will defend the female and his territory from the other males.
And when i say defend i also mean kidnapp and control. My male will push the female into their shell at the slighest sign of danger. It'll take a couple days for your couple to be comfortable enough for the female to come out often. I've had mine for about a month now, and the female is out a lot of the time, but the slightest sign of motion by someone else (lol they get to recognise their owner) and she disappears. Just give them time to settle in.