My 2 cents:
For steelhead you'll likely be using 8-12lb leader line. You want your mainline ideally 5lbs (ish) stronger than the strongest leader line you'll be using. For example, 10lb leader and 15lb mainline is a typical good combo.
Typically most guys run 12 or 15lb mainline of their pin reels. I run 20lb on mine (Maxima Chamelon) as I am new to centerpinning and the staff at the shop advised me that this stiffer line would be easier to learn to cast with. So far I like it, no issues.
I would just take your reel and rod into a good shop (Searun, Fred's, or wherever you typically shop) and get them to spool the reel up with backing and mono mainline. As the mono wears out or you want to try something else you can easily remove it, reconnect to the backing, and re-spool up with whatever. I think next I'll go 15lb MAxima Chamelon as the 20 is a bit overkill for my set up IMO.