Hi Shaughan.
Mate I've kept lizards in outside pits on and off for the past 50 odd years and can assure you that provided the area is large enough with plenty of appropriate shelter sites most of the the larger species of skinks and dragons can co-inhabit without too much trouble. The only restriction on the types would depend on your location and local climate. Personally I believe that it is without doubt the best way to house larger species of lizards especially if you want to observe them do their thing in as natural setting as possible. The only real problems you'd face is that smaller skinks and frogs won't survive if you intend to keep Water Dragons, Bearded Dragons and turtles. Water Dragons and Beardeds will eat anything small enough to get into their mouth and, depending on the species, turtles will also clean up any frogs missed by the lizards. If your intention is to breed critters in the pit just be aware that Water Dragons will dig up and eat any eggs laid by the Beardeds (or other species of dragons), as well as eat any of their own off spring and baby Bluetongues if they are not removed after hatching or are born. Best of luck with your project.
Hope this helps.
George.