Hi there and welcome tot he forum. :happy:
I have attached a link with some information about tank sizes for axolotls
http://sillyaxolotls.wixsite.com/sillyaxolotls/tank-sizes
15g is crowded for 1 axie. I guess it would be ok for very young axies, but you would need to be looking at a much bigger tank very quickly. All my guys are mature now, and are around 12 inches long. They are not the most elegant or co-ordinated of animals, so quite a bit of space for moving/turning, and several hides are a good idea.
The oxygen/gill size debate is ongoing. Whether gill size is related to water quality, oxygen, environment I don't know as I'm not experienced enough to be able to get to the bottom of this, but I do have live plants in my tanks because I like thom and my fellas seem to enjoy hiding and resting in them. I have an issue with some of my fellas having smaller gills at the moment, but this isn't related to the plants because they've had live plants in their tanks for a long time, and the gill problem is more recent.
As for snails inthe tank, as a general rule it is not a good idea to have any other anumals in the tank with your axolotls.
Caudata Culture Articles - Species Mixing Disasters
Snails the same size as gravel will cause the same problems asgravel, as you have mentioned. Fine sand substrate, slate tile, or no substrate at all seems to be the most popular for axolotl safety.
I've only had my gang since February/March, so not very experienced, but this forum and the following sites are those I have used for research.
Caudata Culture Axolotl Articles
Axolotls: The Fascinating Mexican Axolotl and the Tiger Salamander
http://sillyaxolotls.wixsite.com/sillyaxolotls
I hope that this has been of some help. :happy: