<FONT color=#000080><SPAN style="mso-bidi-font-size: 12.0pt; mso-bidi-font-family: Times New Roman"><B><FONT face=Arial size=2>Male and female sunset platy<FONT color=white>
would definately be a nice looking tank (that's 19 fish in a 15 gallon, but I guess they occupy different levels of the tank). I'd personally go with a crayfish, snails, a couple of rams and some hatchet fish, with crushed coral as the substrate, to ensure healthy snail shells.
not knocking your idea though. I just have a 20 gallon growout tank that I want to put some permanent occupants in, and I'm thinking for myself more than anything (I don't see myself buying anymore juvies to grow out any time soon, I'm overstocked as it is).
you'd be lucky if your crawfish reached 5" (not counting the claws). they're big enough to be pretty impressive in a 10"x20" 15 gallon tank. I've heard of them reaching 6"+, but that would still not be too big for a 15 gallon.
just beware keeping a crawfish. they are nocturnal, and hunt at night. they will potentially kill fish sleeping at night (I've never had personal experience with that, just anecdotal). I would only keep crawfish with fish that are easily replaced. he'd have to be the centerpiece of the tank.