There is no query limit - not sure what you mean by that; just as there is no user limit. I've had hundreds of users on my galleries at times with no problems.
When you say short URLs, do you mean like "member.mygallery.com"? That can be done with a combination of htaccess files and/or a php script if you have wildcard dns enabled on your server, but because it requires a specific set of conditions it isnt something that can be done on all servers. We don't support this ourselves, but I know some users are doing this on their setups.
Please do not PM me for support or sales questions. Thank you for your understanding.