For a drive train, would GT2 belts work on HTD pulleys? Specifically, a 5mm pitch GT2 belt from sdp-si.com on the AndyMark 42 tooth pulley. I wasn’t able to find any HTD belts of the right size that weren’t sold out, and I saw that the GT2 belt shape is similar to the shape of HTD belts.
According to Gates, it will work, but is not recommended. They will have a reduced torque rating. They also say not to use HTD belts on GT2 sprockets. The only time we used 5 MM belts on the drive train, we had issues with them breaking. But then our competition driver was a mad man. We changed to 15 mm belts since I couldn’t change the nut behind the sticks. 
What is the right size? Vex has lots of sizes in stock http://www.vexrobotics.com/vexpro/motion/belts-and-pulleys
They also have a great calculator for finding the right size:
http://www.wcproducts.net/how-to-belts/
This does work, yes. I’ve heard that it is slightly stronger than HTD belts on HTD pulleys, but not as strong as GT2 belts on GT2 pulleys. But this does work - 2791 used this method in 2012 and 2013 to power their drivetrains.
The 5mm that the OP referenced is the pitch, not the belt length. Nearly all pulleys sold by FRC suppliers are 5mm HTD, either 9mm or 15mm wide. You were probably referring to the latter?
(Duh) Silly me. We had bad luck with the 9mm wide and switched to 15mm wide. They are both 5mm pitch as you correctly pointed out. YMMV depending on transmitted torque & pulley size.
Somewhere between 120 and 130 teeth. I found belts with 125 and 127 teeth, but they were all out of stock.
Have you tried (http://www.bbman.com/)?
Depending on what width you need SDP/SI has some in that length range, but all their stock looks a little low.
https://sdp-si.com/eStore/Catalog/PartNumber/A%206R25M125090
I’ve used Vbeltsupply.com several times. Also Amazon has HTD belts.
The part number Is length_mm - 5M - width_mm
Your 127 tooth belt would be a 635-5M-15, a quick search on the correct Gates P/N should yield plenty of supplier options.
Has anyone else had issues with belt calculators coming up short on center to center distance? Our belts are a lot looser than we’d like then to be, using the distance provided on the WCP calculator.
Just to double check, does the WCP calculator match the SDP-SI calculator?
https://sdp-si.com/eStore/CenterDistanceDesigner
Proper tension for a belt is surprisingly loose in my experience. It shouldn’t be skipping or anything, but the belts do have a little give. I’ve found teams that manually tension belts tend to over tension which decreases belt life.