I installed a drain in my duck pool, 1/2 inch or whatever the size is of a standard garden hose. The ducks are so mucky it clogs pretty quick and I've found my drain is virtually useless. Depending on how often you are draining and cleaning the whole thing you may have better luck, just something to think about, how do you plan to deal with clogs. You may need an easy access way to disassemble your drain to unclog it; you may not want to silicone the pvc drain pipe to the waterer.
Another thing to think about is filling/keeping it full in winter. Hoses can freeze up pretty quick. I'm not sure where your water source is that this will be connected to and how reliable it is in winter. You may need to make modifications to keep things thawed. My outside hose, and sometimes spigot, freezes regularly and I often have to fill my water buckets in the house in winter. There are lots of clever ideas on here and products out there to help with frozen water, but figuring this out BEFORE winter will certainly help.