I don’t have a dog in this fight but I do know something about fly lines.
Fly lines and Fly Line Backing are two different things.
Backing is either braided or mono line used to take up space on the reel and if you’re fishing Tarpon or king Salmon or the like they might go on a long run where you’d need the extra line. In an ironic twist to the story, I use tip-up line I bought in a clearance bin at FF for my fly line backing.
Fly line is thick and heavy. It’s usually a mono or braided core with PVC over it. Most fly lines are designed to float although they do make sinking lines too. I would think a floating line would the wrong application with a rattle reel. A full sinking line might actually work pretty slick, just drop it down the hole and get down there quickly. You wouldn’t even need sinkers. Most fly lines boxes have a code on the front that indicates what kind of line it is. WF6F would translate to “weight forward, 6 weight, floating”. WF6S would indicate a sinking line. The cheapest fly line I’ve ever seen costs about $30 for a 100 foot log line.
S.R.