I've kept many Tokays in cages with screen tops and never had any problem with injured feet or toes. I did have one Tokay that rubbed its nose raw against a screen top but it healed just fine when it was moved to a new cage. So anything's possible but overall I'd say the probability of an injury due to a screen top on your cage is low.

I keep water bowls in all my tokay enclosures. I have personally witnessed many of my tokays (I currently have 18) drinking out of their water bowls. They also seem to like to defecate in them, so I clean them frequently. But I also do mist daily - just to keep up the humidity here in arid colorado. I's say if yours is not accustomed to a water bowl, I would mist just to make sure it stays hydrated while you're determining whether the bowl is being used or not.

Finally, 18 x 12 x 18 should be ok for a single tokay but IMO is a bit small for a pair.

Hope this helps...