I found a foolproof method. Wait and see who spawns - my newest female just deposited a clutch of 'em She's still small, so it's funny watching her boss my large Bala around. I have the feeling I've got three females and one male, not the 50-50 split I was aiming for.
Off to the fabric and hobby shops tomorrow. I plan to build a tank divider out of fine netting and plastic rods, to let the parents raise 'em in peace and quiet. Thankfully the rocks the Bolivians like to spawn on in my tank are down one end, so I only have to isolate about 15% of the tank.
Not to scale, lol