1. Personally I would spend the little extra and get a T10. Makes cleaning the cage a lot easier. As for dividers, yes I see people do it but honestly I would give the whole cage to 1 snake. If your BP is anything like mine, she will love to cruise around at night and "play". During the day, my BP just sleeps in one of her hides. My girl is about 2 years old and about 4' or a little longer.
2. As for heating, I use flexwatt in my T10s and love it. I do keep my house around 75F though and the basement is actually warmer, probably closer to 78-80F so flex worked great for me. I do have a radiant heat panel in one of my cages though which I personally don't like for warm rooms as it doesn't seem to really create a hot spot without heating up the rest of the cage as well. If you keep your house cool, a RHP would work awesome as it would be on much more and keeping a nice hot spot while the house ambient would keep the cool side cooler.
Don't cut holes in your cage. Just order a radiant heat panel. They are easy to install and just require a drill and screw driver. As far as add ons, I would definitely say get a shelf. My BP LOVED her shelf and would climb on it all night long. Also about the RHP, AP will install the RHP for you if you have whoever you get it from, I recommend either Pro Products or Reptile Basics, drop ship it to them.
3. Well the T8 and T10 are the same length and width. just 3" taller on the T10s and of course the door opening is bigger on a T10. The door opening on a T8 is 22x8 and the door opening on a T10 is 21x10. Trust me, that extra 2" in door height makes a difference when cleaning plus the extra height is nice if you go with a shelf. The total dimensions for either are around 48x24x12 or 15. They are heavier than other PVC cages but they are built strong. I personally love the 3 I got and am ordering 3 more in the spring and then probably 2 T25s next fall depending how big my retic gets, might be getting 3 in that case lol.
Here's a picture of my cages. The white one on the right is a Pro Line from Constrictors NW. And a picture of what the lights they install look like and a picture of a couple of my gals and guy using the shelves.
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