I used to have 2 ADFs (one died from unknown cause =( ) that I keep in my 15 gallon community tank with some small fish, and they were reasonably active at night. Personally, I would go with the 5 gallon. Bigger tanks are advantageous in that there's more room for error, and less maintenance required in general. It may sound odd, but the smaller the tank is, the more work you'll have to do! I have a 15 gallon and used to have a 5 gallon, and the 5 gallon got dirty much more quickly..
As for cycling the tank, the rocks from your other tank would definitely help with the process. : ) Another method that people like to use to speed up cycling is to "borrow" some filter media from an already established tank and place it in the new tank's filter, on top of its existing filter media, since most of the good bacteria is in the filter media.