I've been playing my soprano ukulele for a while now and I'm realizing that it's much to quiet to play along with the average acoustic guitar So I've been wondering what sort of pickup would work on a soprano ukulele with nylon strings
A piezo-electric pickup would work best. If you don't have metal strings, you can't use normal magnetic pickups.
My father custom-made an electric pick-up for his violin way back in the 60's, and it worked well! It was not a magnetic type - and I am not exactly certain how he made it. Guess I'm just telling you to let you know you can probably make one special for your instrument that'll work.
I don't have the resources to make things like that, I live in a little mountain town and pretty much anything useful has to be ordered off of the Internet But I did happen to find an affordable piezo-electric pickup on eBay that specified that it would fit on a ukulele
You can make a simple pickup out of a piezoelectric tweeter, like you might find at Radio Shack. Ever seen those Dean Markley acoustic pickups, with the round wood button and guitar cable? Same thing. You have the red and black wire on the tweeter, that's the center and ground to the amp input. I made one and affixed it permanently under the guitar with a piece of putty. I put epoxy all over the wires and stuff, so they wouldn't pull out of the transducer if I had to move the pickup around. Then it was connected to an endpin jack. To make a Dean Markley type you have to wire the guitar cable to it and make something to encapsulate the transducer, something like Build 50 could be made to work. Good luck!