The official line from the Instructors at SFC is.. if you can see through the rain and see the horizon and the surface and it is in patches.. then you can fly through or route around the "light rain".
Minimum Cloud base - there isn't one.. it is depending on what exercise you are doing.. so for general handling i would say 1800 feet - stalling 3000 feet etc.
Check with your FI.
Scoobs.