You can use #20 test, I've used #30 coated multi strand which is the same dia as #17 but they have kinked for me and I just prefer to use #17 mono as I have alot, that's what's on all my trolling reels. The only problem you should run into would be there are alot of harness hooks where the eyelet is too small to run even #17 test thru for snelling. I switched the hooks I use and now have no problem, I just have alot of hooks I'm giving away as you can not get the mono thru 2 times thru the eyelet.