I used to have a converted swimming pool reptile pit when I was a kid and it was awesome.
It does depend on what kind of pool you have to start with though- is yours in-ground, half/half or above ground? A concrete or fibreglass in-ground pool is probably not going to be a good pool to start with.
I had a half inground/ half aboveground pool that was just lined with plastic so we ripped that out and had a dirt base. We made a mound of topsoil in the centre on top of some rows of Besser bricks covered in tarp and also a few lengths of pvc pipe to make some dry tunnels in it. Used one of those kiddies plastic half clams shells sunk into the floor as a pond for water that also housed a turtle. Planted it out and put all the raked up leaves from the garden in a pile for the bluetongues to hide in, and topped the mound with large rocks and hollow logs to make crevasses and solid basking sites (make sure the rocks are really secure though).
About once every wet season, after about a week of monsoon rain I would have major dramas with flooding in the pit due to the ground becoming waterlogged and unable to drain away any more water. I'd have to evacuate everyone into pillowcases in the spare room for a few days untill the water went down (except the turtle, who thought it was fabulous). I'd be out there in the middle of the night with mum holding a neatly folded pile of her pillowcases, a torch and an umbrella, and me thigh-deep in muddy water in the rain grabbing bluey's all piled up on the very top of the mound or clinging to a floating log or the spindly tops of bushes.
Sounds a bit disturbing but I never ever lost any animals due to flooding in the pit as there was always high ground to go to. The mound in the pit was tall enough to be a little higher that ground-level outside the pit, which I think helped a lot.
I was living in Darwin at the time so heating wasn't an issue at all. I kept a heap of bluey's, a couple of monitors, a hosmers skink and a long-necked turtle in together, and the only problems I had with them together were the blueys fighting in breeding season. I think the trick with your animals is to keep to species that occur in areas with the same climate as yours so they'll be ok outside.