I've got PVC nipple waterers with the thicker PVC. Some of the nipples are screwed into the PVC cap if they are an upright version, which is really thick.
It should work with Sch 40 PVC. Make sure you use a wide enough diameter PVC tubing that the threaded part of the nipple waterer has room to fit inside w/o rubbing; at least 3/4 inch? 1 inch?
And make sure you use the exact drill bit size they recommend. Then, you could put some plumber's tape on the part you screw in, along with silicone caulking right over the top. Or, just the silicone caulking, but either way, smear the silicone caulking generously over the threads of the nipple waterer.
Then screw in firmly. Let this dry overnight.
I hope this works!
The first one of these I built, I made with 3/4 inch schedule 40 pvc. The nipples went in with difficulty despite using the correct drill bit, and they leaked. We sealed them with silicone and they don't leak anymore - but your suggestion of going to a bigger pipe I think is the key if we use schedule 40 pvc. I have to build another one of these and I'm going to 1 or 1.5 inch pipe for the nipples and if I can find it, I'll make the tank out of 6 inch.