combining species is a sore subject for most people. i personally don't have a problem with it and have never, and i mean never had a bad experience with it. there are two primary things that you must consider for this to work though.
1- the species you are mixing should be from the same geographical region with the same or very similar habitat needs,i.e.- lighting, temps, humidity, etc.- food is not included in this as long as they are willing to accept hand/tong feeding of any special items you don't want the other species to try to eat.
2- make sure that the different species grow to be about the same average adult size, and that when you put them together, that you don't put adults of one species with babies or juvies of another species.
as for your wanting to mix a RETF and a giant leaf tail gecko, look at the name of the gecko. i have seen adult Uroplatus fimbiratus and they grow to be quite large, many times exceeding 12" as adults. RETF don't grow large enough to not be considered a food source for an adult giant leaf tail. so i would say that this perticular combo is not a good idea, but you may find adult whites treefrogs as a better choice.
as for my mixed species tanks:
30 gal. high- 2 golden treefrogs
1 green treefrog
1 grey treefrog
1 squirrel treefrog
1 southern leapord frog
1 blue spotted salamander
this tank is terrestrial/aboreal with a naturalistic vivarium blueprint using all live plants, branches growing live moss and lichens, moss covered rock caves and six inches of forest bed substrate for burrowing for the salamander and occasional treefrog or leapord frog. even with the high toxicity of the blue spot, i have never had a problem because the other inhabitants pose no threat to the well being of the salamander.
35 gal hex- 2 adult whites treefrogs
1 adult new caldonia crested gecko
1 adult gargoyle gecko
aboreal set-up with tall live plants, real branches siliconed into the tank in different levels for climbing and sleeping (which is done alot) and agin, as all my set-ups are, it is a basic style naturalistic vivarium with all live plants and real wood.
29 gal long- 5 korean fire-bellied toads
1 european fire-bellied toad
2 fire-bellied newts
this a semiaquatic habitat, with 2/3 water to 1/3 land, seperated by a plexi-glass divider cut and siliconed into place. the water houses multipul rock caves/platfroms for hiding as well as climbing. arum and pothos are planted into the gravel bottom and a whisper 40i internal filter keeps a constant flow of filtered, moving water in the tank. the land portion as pothos, arum, birds nest fern and creeping pepermonia palnted into four inches of soil, which is coved with sheet moss to help keep the soil of the toads and out of the water.
75 gal.- 1 barred tiger salamander all are confirmed
1 blotched tiger salamander to be diff. species
1 eastern tiger salamander
1 airzona tiger salamander
1 yellow spotted salamander
this is a terrestrial set-up that ends up also being subteranian at the feet of the salamanders. it is styled after a temperate forest, with a large rock cave formation coverd in moss, a bark cave that acts as a fallen tree for cover, and many different plant species providing cover and looks. there is approx 6-8 inches of forest bed throughout the tank allowing for plenty of digging and tunneling space. a large water bowl for the occasional dip is also provided in every tank. even though the tigers are not all from the same area, and evn the same identical habitat, they still are able to thrive quite well in a habitat that attempts to mimic that of the northeast.