Spike
I have a two different sink rate Rio dc 15' sink tips. I've cast them on 4 different rods including a $59 Redington, an original echo series, an Orvis T3 to an old Sage RPL from 1990. All actions very different. What I'm getting at is you don't false cast keeping 50' of line in the air while you search for a target. Casting is way different than what I consider normal fly casting.
You strip out a pile of line (40'- 60'). Shake 10' feet of the line out of the rod tip, take your casting back stroke, haul and it shoots all your line out. It's awkward but amazing. Try to false cast like you've got a #16 sulphur on your line and you'll be picking the streamer out of your scalp....or......the sink line and fly will strike your rod blank snapping it in half. The best description I can give is the back cast is like fly fishing. The forward stroke is more like a spinning rod cast.
Just about any roe will work if you can adapt your timing. Where do you plan on smallmouth fishing? If you are looking to bomb it on the Susky, a 9-10' rod in 7 wt may make it easier. I find a stiffer rod gives you the ability to lift more line off the water to start the cast.
To answer the question you originally asked.....
I think a 10, 12 or 15' sink tip should be pleny long. Cabelas has some of their lines in the bargain cave for $20 or so. SA make a streamer express, Rio makes a streamer tip. They did make sink tips in their Mainstream and / or Avid lines but don't know if they still do. You'll see everything from intermediate to 10" per second rates. You have to consider the depths you want to cover, the water speed and if you'll be tossing tube flies or sculpin helmets. In the Susky last summer, I tossed a 7ips with unweighted butt monkey or sex dungeons. Very few snags but the water was up and I was doing it from a boat.
If you can find an old Rio density compensated (DC), the rip is uniform sink. That means you can trim it without hurt the castability
