Pumpkins and pototoes are mortal enemies of each other LOL. Seriously, they aren't companion plants so you have to keep them apart otherwise they wont grow properly. If they were sprung from compost scraps, then I would say that they are not fertile. The plant will still grow just fine, but you are unlikely to get any fruit as the original plant was probably a hybrid or bred as a single use plant and any saved seeds would be sterile. If you do happen to get a fertile seed self sow, any fruit can be a bit hit and miss in quality. I remember a few years ago FIL had a pumpkin plant shoot in the chook pen and it grew the most bizarre looking pumpkins but they were 100% edible.