Neons are probably the easiest of the phelsuma's to keep I think personally - no aggression towards each other and the babies are just fine and can be left to grow on with the parents in the same tank.
Couple of words of caution - these guys are real escape artists (particularly the babies); if you plan to leave 'junior' in with Mum and Dad (or buy youngsters) make sure you hand pick the Exo and check that there's a tight seal at the edge of the doors (if a fruit fly/small cricket can get out junior can get out)! Also on Exo's you'll need to seal the holes accross the back where cables etc go in - even with the slots shut I've known adult neons squeeze out - I normally just use blu-tac as it's not permanent and the geckos don't get stuck to it! (if you want a couple of photos of how I've done this let me know) - other option would be to go for something like a Lucky reptile tank which is a different design, (no idea how much they are in the States, they're more than the Exo's in the UK), but a lot less escape avenues for the geckos!
Only other thing I'd say is if you're going for a 18x18x24 I'd get more than a pair (they're a reasonable amount smaller than gold dust) - probably something like a 1.3 - they're a very sociable gecko, I've got a group of 2.6 together in a 24x18x24 and they all get on fine - even the males completely ignore each other, either that or just put a pair in a 12x12x18; if neons have too much space they'll all just congregate in 1 area and don't use the whole enclosure from my experience.
Other things you'd easily get in that size Viv would be a pair of Pea****s - P quadriocelletta, a pair of lined - P lineata, Mauritian blue tailed - P cepediana (these are quite shy though), any of the smaller phelsuma basically. There's a very good book - 'Professional Breeders Series - Day geckos' (I've just checked, it's on amazon.com for about $28 ). If you don't have it it's well worth getting a copy - there's a good section with species accounts, and general husbandry and breeding advice as well as a lot of color photos so you can see what different things look like.
