Okay I'll try and answer a few of these. Def not an expert but we've had several of them before and our current one for seven years now.

I would not put a baby in a large cage. A smaller cage would be better IMO. A good adult size is 36" x 24" x 24" or even a 48". They are arboreal, but do better in wider cages IMO then taller cages. They will move left to right on their perch to regulate their temps. We use a radiant heat panel for heat which works perfect for their setup. (high humidity and wet at times)

Babies feed on pinky mice and will grow fairly slowly compared to a lot of other species. Our adult GTP is just about six feet but you would never know that bc she's always curled up on her perch (or sometimes the ground.) She eats a small rat usually about every ten days, but that really depends on her mood. Sometimes she will not eat for several weeks before she decides to eat again.

Handling them will depend on how aggressive your GTP is. The easiest way to move them is to have removable perches/branches and just remove the entire branch when you need to clean or handle them. Some will tolerate handling (our Aru has never struck) but many will not and should avoid it. A snake hook will work well for this if you have an aggressive one.

No clue on what is better a male or female. We have a female and have not had a problem with her. If you buy a baby you will probably not know the sex at purchase. Red or yellow does not really matter if you are just looking for a fun pet. There is a lot of debate on that however as far as how the color matures. I would buy a CB baby vs a CH.

As for types, that will also very greatly depending on their lineage. In general, Biaks tend to have more yellow and green coloration, an Aru is more bright green with white and blue markings, and Sorongs develop a nice blue stripe. It really just depends on the lines though as I have an Aru with more blue on her then a lot of Sorongs.

Hopefully that helps