Claudia - I had to laugh at your description of oats as a 'safe feed'. Ha! It may be good for their gut compared with some cereal feeds, but my horse turns into a raving loony if he gets a sniff of them (or any other grain, for that matter, pelleted or otherwise). He sneaked into the feed room one day last year and pinched about a pound of oats from another horse's feed before I found him - for three days, I was too chicken to ride him. He spent most of this time on two legs or running around screaming! Oats? I don't think so...
Alison - if it's possible, could you put your mare next to a fairly passive horse in the stables for a few days, then turn them out together? If they have a chance to form a pair bond in the safety of the stable, then they'll probably be okay turned out together. Also, if they've formed a pair bond, they might even be okay turned out together with the other horses. If you're really worried about your mare hurting one of the other horses, could you take her back shoes off and use boots if you need to ride her on hard surfaces? Used to do this with my horse, who shared a field with a show horse for a while.