You should be ok with that timing. Just have in mind that AA arrives in Terminal 1 and RG leaves from the newer Terminal 2. It's not a long walk, you'll need to get to second floor (departures) and then maybe a 10-minute walk in slow pace. Last time my American friends were here they said Customs was a piece of cake, but I guess that can vary a bit. Maybe if you have time in MIA you should drop by RG's counter and inform them of your conexion, they might even be able to check you in.