I realize that electric and glow are 2 completely different methods of powering models. I also know that electric suggests using larger diameter props to achieve similar performnce. But here is my question. If a larger prop can not be used can more blades be used ? Could an electric 1/2a use a 5 or 6 blade propeller ?

Yes, I realize that the 6" rule in your case was a local rule. My issue with bigger props is that as far as scaled down full sized stunt planes go,  props that are greater that 6" start to make the landing gear look wrong to me. As far as making a mock up of a 3 4 or 5 bladed prop I could make one easy enough . Though I doubt i'd be competent at making  a mold for it.