I am not irrigation expert so I only know the basics: OSPi works with standard 24VAC sprinkler valves. These valves generally come at different fittings — most of them use NPT, national pipe thread, fitting, with varying sizes such as 1/2″, 3/4″, 1″ diameter. You can choose them based on your pipe fitting. If you need to connect them to garden hoses, there are NPT to GHT adapters you can use.
Wiring wise, say if you have 10 valves, each valve should have one wire that goes together and that’s the COM (common) wire, which goes to the COM terminal on OSPi. The other wire on each valve goes to an individual zone port on OSPi.
I don’t know much about backflow prevention and stuff like that.