I've kept dwarf puffers before and they are the cutest, nippiest little beggars around!
I'm not sure how many you could keep in a 15G but my guess would be 3-4. the tank needs to be heavily planted and have plenty of hiding spaces as they will become quite aggressive with each other, especially at feeding time. It might be an idea to start a new thread and get opinions from other members who have kept them.
As for the shell-dwellers, they are relatively easy to keep. I find them no more difficult than keeping other community fish. They have great character and i could sit watching them for hours. They like to hollow out the sand around the shell so that the shell sits on the bare glass. They either scoop it up in their mouth and spit it out a few inches away or flick the sand out using their body.
I know it's a tough decision but I'm sure you'll work it out in the end!