I put the plexiglass inside the tank, resting directly on the bottom glass, prior to adding gravel or rocks. Then I place the rocks so they're resting stably and sitting directly on the plexiglass, then I add substrate. (I don't want rocks resting on sand or gravel because as the fish dig, the rock piles will tumble.)
Some LFSs will sex the fish for you, others won't. I'm fortunate in having a place that will do so upon request. I'm not sure I'd buy so much from them if they didn't. Some mailorder places will sex them, but many will not. If you can't get them sexed, there's not much you can do, unless you learn to do it yourself...
Good luck,
Jim