1200rpm

i am having CM ELITE 310 cabinet with a rear 120mm fan 1200rpm i want to buy another 120 mm for side intake. 1)should i buy 120mm,1200rpm,39.77 cfm OR 120mm,2000rpm,90 cfm? 2)will two fans of different RPM and CFM cause any disturbance in airflow? 3) can i use them like 2000rpm for...