Applesnails with goldies is tricky business. Lots of times the goldies will like to nibble on the snails, nipping off tentacles and sometimes eyes. My goldies leave my snails alone...but I've heard most don't. If you decide to get a snail have a back up plan in case your goldie sees the snail as something tasty.
Dojos are cool critters and can be kept with goldies as far as I know. I had a pair at one time and they got along fine. The only thing I don't like is that you can't use a lot of common medicines with them because they're scale-less. The same is true for snails but its easier to catch and put a snail in another container when you're treating the tank. Dojos are hard to catch. They also require food other than goldfish food (so do snails). The worst thing about dojos (in my opinion) is that they can jump out of the water...if your tank hood has ANY little hole in it (even by the filter) you might come home to a dojo on the floor. This is a good site for an overview on dojos:
http://animal-world.com/encyclo/fresh/loaches/DojoLoach.phpFor applesnails this is a great site to check out: www.applesnail.net