Is it okay to tell a mentally handicapped customer to calm down?
Are you worried about dealing with mentally handicapped customers?
no, I've had experience dealing with the mentally handicapped before.
It is fine to try to stop the customer from getting angry, especially if they are handicapped.  You and the other people around should also be understanding about their frustrations.
Shit, I think it just sounds like I'm discriminating against them.
It could be construed that way. You should try to be more lenient with them but also realize that they don't get to be rude and disruptive in public. Demonstrate kindness towards them and they'll reciprocate it back.