right...
As for sexing them, acording to PlanetCatfish, males grow elongated interopercular odontodes during the breeding season. Males develop noticeable odontodes along the dorsum as well.
Clowns are aparently one of the easier plecs to breed.
You'll need a group of 5 (give or take).
They will also do a lot better in a species tank so you can get everything how they like it and the wont be hastled by other fish.
30 gals is a good size.
you'll then need some where for them to spawn. (tight caves or small plastic tubes will do nicely)
there are loads of sites out there for advice.
(PS - i've not bread them by the way - just read up a lot and most plecs are the same - eg our bristle noses and our columbian zebras both have similar habbits)