Can I do really anything in Aldria Open Mush?
No. There are certain rules and limits that have been set now, and as time goes on, more will come. But those will not stop your characters from going around the world and do as they please within the limits of that world. You can become a pedlar, thief, mage, general or even king, you just need to get other players to work with you towards achieving your goals.
In short: try to make the experience pleasant for everyone on the mush, especially your fellow players and you'll get away with a lot.

Can I truly become any race I wish?
Yes, but again with limitations. You'll need to make sure your race concept is well documented and that its history fits into the current known history of Aldria Open Mush. Admins will review this info, but they will rarely stop you and might even give suggestions to make your special race work.

Can I build myself a house and such right from the start?
Right now, no. We're looking at various possibilities to enable players to build what they wish where they wish to after a short admin review, but right now that functionality isn't set yet. So, while admin is limited in time, they are the ones you should talk to when you wish to build yourself a spot in the world to call your own. Do try to stick in character while building though, don't make a castle for your beggar.

Where can I find more info on what's already set and done?
You can find limited information on this site and the mush itself, while we work on getting a Wiki up where all information will be gathered and players can submit their ideas for admin review.