A 30 long would be good enough if you take out all but 1 male when they get to breeding age.
And if that's a good lfs they should be able to vent them for you. Or let you do it.
Or this is what you can do.
It's not 100% accurate but it bumps up your chances a whole lot.
Pick out about 8 of the smallest yellows from the batch that has VERY little or NO black on the dorsal and anal/vental fins. (Most likely they'll be females) And 2 of the biggest from the batch with the MOST black on the dorsal,anal/ventral fins AND pectoral fins. (Most likely they'll be males.)
~Hype