Neither cooler (Carrera or RX7) will fit in the opening provided by the RS front bumper unless you perform some front sheet metal surgery. I used an Earls cooler (#25012) which fits behind the bumper w/o any sheet metal mods. The dimensions are 7" wide x 2" thick x 15 5/16" long, enter/exit one end. If you're using -16 lines, use #25016. The fit is tight with hardly any room for exit air. Still, my oil temps don't go over 210ºF (w/rs type 2.7), but that's not in a racing situation. I used -12 braided directly to factory oil lines. Not sure about installing -16 hose which requires a larger bend radius.
Plumbing a front mounted cooler in series with your Carrera cooler is a good idea and will give you more cooling cushion (not sure what engine size you have). You'll need a metric-to-AN adapter (30mm to -12 or -16) to go from the factory cooler to the -AN cooler. You can get it from Pegasus Racing. Pegasus also carries stainless steel mesh which is stiff enough to ward off rocks and most loose car parts. Look at Summit Racing for the AN stuff and Earls - usually good prices.
Hope this helps,
Sherwood Lee
http://members.rennlist.org/911pcars