Go to your local pet smart and see if they have a tank of them,Every one Ive been to in Norcal has them in stock.
Pick the biggest one in the tank and it will most likely be male, esp. since chances are they will be from the same brood.
Also somewhere around 5" the males start to develop quite a bit more freckling on the sides. (and the pet smarts around here usually carry around 4-6" snooks)
If I took my own advice I'd live on a cot in the middle of a fish room warehouse. I agree with PeanutPower. And if all your wanting to keep with them is severums, I'd definately do it. I've got a male, 7ish" now, and he's one fish I would never regret getting. I'm about to go up in tank size soon, and am considering getting him a nice lady friend
I think that's a good combination because I have 2 severum and a cat in with mine, and they along with my bichirs are the only other fish he doesn't get territorial with or even acknowledge are in the tank.
I personally don't see a problem with it, as long as you're well filtered (in the long run). Your male will be the biggest in the tank at up to or around 20". The female won't be as large, being quite a few inches smaller. One thing I don't know is their temperment while spawning. Someone who's spawned them might chime in, but that would be my only true concern. As for the cat, I stayed small just for the filtering concerns with a four line pimelodus. A lima would just push the filtering that much further.