To start you off, I will mention that the guy who runs this site also runs Axolotls: The Fascinating Mexican Axolotl and the Tiger Salamander, and Caudata.org, which will be great places to go for reliable information and to ask questions along the way.
I'll try to answer the questions you have for now:
One. You're right that there's a debate going on over exactly how much room they need. Because they poop a lot and get quite large, a 10 gallon tank would be an absolute minimum, (and some people would suggest that you go larger to ensure your axolotl has enough room to move around).
You've read correctly about gravel. The rule of thumb is that, if it is smaller than their skull, they can probably ingest it. The problem with rocks (once you go that large) is that they trap debris (i.e. poop and food), and so it's harder to keep the water clean. For this reason, most owners go with bare bottomed tanks or sand.
Live plants will be fine. Unless firmly secured, there's a chance of uprooting, but if you go for the easy-to-grow, hard-to-kill species, you should be fine. (And many plants can grow floating, so that's a plus). Plastic/silk are ok as long as there aren't any sharp edges.
The main concern is to keep the tank current-free, as axolotls prefer still water and are stressed by currents. There are a number of ways to do this, so poke around the sites I linked above.
Once they're mature, males will have more of a bulge than females.
In a ten gallon, you really won't be able to keep two.
Have fun searching for more info.