I'm going to be one of those people that says mixing species in one enclosure is usually not a good idea. For the most part, the husbandry needs and requirements of multiple species do not match closely enough for it to be a good idea to keep them together. Then there are risks with one species eating another, dominating another causing it to fail to thrive, and one species passing parasites on to another species that is not able to withstand the affects of the parasite (certain species can live with a particular parasite, while that same parasite will quickly kill another). Those are just a few reasons not to mix species.

Large bodies of water that are deeper than the gecko is off of the ground are generally not a good idea for cresteds. They pose a definite risk to young cresteds--you shouldn't even have a deep water dish in the enclosure if your gecko is young. I don't use anything bigger than a 3/4 oz deli cup for hatchlings and juvies--that way there is no risk at all that they might get in it and risk drowning.

The only things that I know of that have been successfully kept with cresteds (and it has been documented that this has been successful) are Australian pink tongue skinks and African giant black millipedes. Cresteds will eat baby skinks though, so you would want to get adults or skinks that are close in size to the cresteds.