The firmware user manual is available here:
https://openthings.freshdesk.com/solution/articles/5000716364-user-manuals
OpenSprinkler uses software defined master zone. So if you enable 1 master zone, you have 7 zones left. You can set up the firmware on your RPi and try out the firmware features without OSPi. This will allow you to test the programs and see if they do what you need them to do.
Regarding your questions about serial vs. parallel, and rain delay, I suggest that you take a look at the user manual first. Your questions may very well be answered there.