why? im no expert maybe its wrong for obvious reasons, the snake is not a constrictor and only eats crickets and bugs right? i kept crested gecko and the small species of green tree snake together a few times, a local reptile retailer said it was ok if a little risky and they were fine for two years? is the deep water the issue?
Green tree snakes are constrictors, firstly. If they are the ones I am thinking of, as green tree snake is a rather generic description, then they are Boids which means they are constrictors. That said, most snakes actually don't specialize on bugs. They can and will snipe your gecko. Another issue is that mixing most species will almost certainly lead to conflict. Cresties are awful swimmers with a habit of falling, as basically all of the Diplodactyline geckos have comparatively crappy sticky pads to work with. Yet another issue is that members of the Rhacodactylus genus, which contains cresties, are notorious tail nippers and eaters.
Probably the most significant issue will be varied heat requirements. Cresties need about 65-70F (basically room temperature), anything more than about 75F can be lethal to them, compromising them very rapidly. Green Tree snakes typically need about 88F in the basking area, which is definitely going to increase ambient temps to uncomfortable levels for the crestie. Not many snakes come to mind that thrive in exclusively room temperatures. Of those, Soloman's Island tree snakes tend to do alright but are obligate lizard eaters so happen to be a profoundly poor choice for mixing with lizards.
Mixing snakes with just about anything else is usually a poor idea, even mixing snakes in general can be a very poor idea. Hell, I've mixed straight up cresties (I used to breed this species) and had individuals that refused to play nice with literally any other crestie. Straight up, don't mix species unless you know exactly what you are doing and have the room to do it. What you are proposing will lead to animal death be it from wild temperature differences or outright physical attacks.
Also as far as roaches are concerned, you have to be very cognisant of humidity and heat with all of the tropical varieties. I bred false deaths head cockroaches and have 4 entomology classes in my university degree I know a fair bit about it

. You are probably being overly ambitious. My suggestion would be, depending on what kind of fish you are getting, maybe look into tree frogs and poison dart frogs. It all really depends on your starting point, I'm more than happy to help you figure out that stuff if you have some patience.
