The "heaviest" as you say, is actually just put on the reel to show you how much line it will hold at that size - for example - 140 yds of 10 lb test. You can put whatever you want on it - the reel won't know the difference (unless you try to go tuna fishing with it. :lol: ). On one of my smaller spinning reels, I've got 30lb braid (although it's only 8lb diameter), and I've been known to go up to 14 and even 17 mono on occasion - but that is not the norm for me. That's a "special situation"... and those don't come along often. Normally, I fish anything 14lb and above on a baitcaster. Not that the spinning reel can't handle it - but it starts to come off the spool like barbed wire after a little while on the spinning reel. 12lb test should be fine. However... which Cajun line did you buy? Cajun Red Lightnin' is made for baitcast reels. Cajun Red Cast is their "super smooth", designed for spinning. Red Lightnin' was too stiff for my taste on a spinning reel.
All the best,
Glenn