When I used this cage for my chameleon, I used a dripper on top for humidity. The cage also comes with a hard bottom that you could place on top to keep more humidity in the enclosure.

I guess you could use it with a snake.  It seems fairly escape proof, and durable enough (assuming it wasn’t a large boa, etc.).  However keep in mind this enclosure is much taller than it is wide or long.  More for reptiles that need vertical climbing space (chameleons for example).

You should be able to mines was only about 3-4 months old when I got her 

No. The bottom piece is very flimsy and will fall out of place, allowing your pet to escape. Get a glass terrarium.

This wouldn't be the best option for a crested gecko, I have personally tried and it poses the challenge of keeping humidity where it needs to be. I think your best option is to look into the exo terra glass cages. They are great for crested geckos.

it is 24x24x48

As a juvenile yes, but after 1 year old or 2 you need a larger cage. They call the larger one "Iguanarium" or you custom build a larger enclosure. Iguanas can grow up to 6 feet in length and need space to move around and climb. Please, do some research, Look for iguana care, iguana diet, iguana enclosure, etc. online or in you tube. Make sure you buy heat lamps, UVB lamps, an humidifier and thermometers and hygrometers unless you are in a warm, humid climate and you can keep it outdoors. They need heat and humidity but also a cooler place where they can hide from the heat when needed.

Absolutely

Yes it’s a great cage. I used mine for a panther chameleon.

I am not sure on the gallon conversion, but it is 24"x24"x48". It is a pretty big screen cage and you could easily fit 20 anoles in there. It is a pretty great cage.