We have a guide to DFW in the American Airlines forum that includes discussions of Minimum Connection Times. For domestic to domestic, it is 40 minutes. In case that you miss your connecting flight, AA should protect you on the next flight to MIA; that, however, is subject to space availability on the next flight from DFW to MIA. AA also flies nonstop from DFW to SCL, so that's another option for protecting you in case that you miss your connection.