I like to use standard fly line backing, 20 or 30lb, with about 75-100 yards of maxima 20lb ultragreen connected by nail knot. Backing is better for the reel as most have posted, it's what Islander recommends, and it also saves you on line as there's no point wasting a whole spool of ultragreen on it, especially if you like fresh line on your reel.
If you haven't fished a CP before you're probably going to get a lot of line twist, and will be wanting to change the line more often, so you might as well save some there. I use leaders in the 8-15lb range depending on conditions, so if i do get hung up it will break before the 20lb mainline does. Combined with short floating, I rarely lose floats, and replace them 90% of the time from busting them on a missed hookset or on rocks while casting.
The new searun is smaller than the IS so I'm not sure how much backing you'll need on it, but basically you'll want to make sure there's not so much line and backing on the reel that it ends up stopping the reel by bunching against the bottom of the reel seat. If you're not paying attention when reeling in a lot of line, it's possible to stack the line on top of itself all on one side of the reel until you can't reel anymore. It's even easier to do if you overspool, and not something you'll want to deal with when fighting a steelie.