I’m no expert on the best size for the soap lines, but i would agree with Alex that the amount of soap used is usually far less than the other parts of the solution, so whatever valve you use needs to be able to accurately deal with comparatively small amounts passing through it. So a 1/2" or even 3/8" valve might be better than a 1"… again… uneducated guess here. If a 1" valve can portion it out correctly and easily, then i guess it doesn’t matter.
Also, i would suggest maybe just focusing on a 3 valve solution for now, for simplicity’s sake, to get a base model. After that, one could look into how adding more valves would work.
Just curious, is there a big price difference between 3/4" and 1"? I would think that even 3/4" would be a big improvement over 1/2", but if there isnt a huge difference in price, and there was no loss in performance from going as large as 1", then i guess it wouldn’t matter.
Here’s a photo someone posted on a Facebook discussion about larger sized metering valves. Dont think you have posted these, but not sure.