Slate is safe for fish.... As for driftwood, does it really cost so much??? I've got a heap here.... Care to make a trip to Sydney Australia to collect 'em?

6" convicts ain't convicts or maybe convicts fed on steriods or something. They're not that big, though the odd one may get to that size.. I've just never seen one.

24" pleco.... err, that's probably the upper limit as well.

Well, the largest Oranda goldfish in the worlds (Bruce) is 24inches long.... Now would that be indicative of how big most Oranda goldfish would get? I'd say 6inch-8inch would be a good size for Orandas and well, Bruce is just a freak.