I don't think the mollies or shrimp would last long at all with any Necturus. N. cf. beyeri might be small, but they can still easily eat these other animals.
I don't know how big a 29 tall is, but your best bet would be a regular 29 or a 20 long, as Alex suggested.
Given that you're in Wisconsin, I think it might be in the best interests of yourself and the animals if you would look into finding a local N. maculosus instead of cf. beyeri from a wholesaler. In my opinion, it's better to collect an animal or two yourself from a healthy ecosystem than to pay a wholesaler (where it's possible that diseases may be picked up, due to the volume of animals they go through) and a collector who goes out and harvests and sells every animal he comes across. If you're really set on a Necturus spp., maculosus might be a bit bigger, but one can fit in a 29 gallon or 20L tank comfortably; I have 3 small adults in a 20L, and a larger group of three in a 75. I'd keep it down to a single animal until you've had success with the one for a while, though.
Given that the walleye run might still be going on in Wisconsin right now, you might be able to post a wanted ad at a baitshop; fishermen catch them frequently during this time. You'd have to nurse him back to health and it might not eat for a bit, but it would be an ideal animal to remove from the wild; a hooked Necturus would have reduced fitness and any care/protection you could give it would be more than it would get in the wild.
