Size is Totally Realistic
An average length of 25 feet is easily achievable in snakes, as Anacondas regularly reach 18 feet in length, Burmese Pythons reach 23 feet, and the extinct Titanoboa reached over 42 feet. As for weight, anacondas can weigh up to 400 pounds, so that’s about what you should get for your snake dragons.
Gliding is Possible
If your serpentine dragons weighed as much as a Burmese Python or Anaconda they would be far heavier than any gliding animal that currently exists, as most of them are but a few ounces. But this does not necessarily mean that you can’t have a 400+ pound glider. Birds of prey frequently resort to gliding flight, and it’s thus very likely that the massive Quetzalcoatlus Pterosaurs were also able to do it, and they weighed hundreds of pounds. The Dragon would definitely have to climb to an elevated position before it could glide.
Extended Ribs and Paddle Tails
These have already evolved in snakes, it’s completely feasible.
Climbing Scales Probably Wouldn’t Work
The scales and supporting muscles would be absurdly strong to propel a 400 pound snake up a tree, especially given that the snake already has a much more straightforward method of climbing; it will simply use its massive strength to coil and move up the tree. Pythons already climb this way.
Having “above average intelligence for a snake” is a very broad field that lends a lot of slither room. Intelligence is a trait that can be selected for, and it could be explained in simple evolutionary terms as simply being more advantageous over time