I'm sure the gurus will be along soon to tell you for sure.
But typically it's advised to use a gate valve on your main drain from the Bean Animal overflow. Gate valves are wicked expensive, but highly recommended here. Other drain lines can be open, but it may be useful to have a ball valve on them.
I don't think there's a hard number or rule for unions, people just suggest that you place them pretty much everywhere, which is a bit vague. Maybe someone else has a better idea.
A lot of people say to use two 45 degree elbows instead of one 90 degree elbow to reduce head pressure, but I believe testing has shown that to be a bit overhyped.
Check valves can be either loved or hated, and again, very expensive. Some use them, some don't.
I haven't plumbed a tank in 15 years, but planning one now. I ordered my gate valves from
BRS, but I ordered pretty much all my other valves, fittings, unions, etc. cheap from the big orange online bookstore. They had good reviews, and if they all work save me a ton of money comparing to buying it all at
BRS or Home Depot.
Here's a good thread some some more info and pics:
https://www.reef2reef.com/threads/show-me-some-diy-sump-plumbing.680593/
