I second joining Lil Beginnings Miniature Horse Forum, a great forum for Mini people.
I also second that its highly advised to keep your Miniature Horse seperated from your full sized horses. Even gentle horses who get along just fine with the mini can spook and run the little guy over some day. A ill placed playful buck can end up very, very bad etc.
NEVER leave a halter on a Miniature Horse while un-attended. They are to light to break anything, there are many Mini hanging horror stories. I used to breed a few show AMHA/AMHR/WCHMR Miniatures and sold a beautiful filly to a nice couple. Within a few weeks I got the call that they had left her halter on while she was "new" to their farm and she hung herself. Very heartbreaking.
Do not use Miniatures for riding horses. They are not cute childs riding mounts.
Minis are however a blast to drive! Miniatures are typically sweet, easy to train and are honestly fun to have around. I know a number of gals who had taken enough riding spills so they got into Miniatures to take "trail drives" instead. They are also fantastic to trick train, that seems to entertain just about anyone.
They are super easy keepers as I'm sure you've guessed. Grass hay and NO pasture does them best. A little handful of a low sugar feed, some BOSS and a little beet pulp was what I fed ours to help keep the bellies off.
Do some research, you'll learn more than anyone here can tell you in a single post.