Barbless wrote
Using sink-tips and sinking lines can get complicated. I'm still on the early part of the learning curve with them.
And the phrase "learning curve" is weirdly appropriate, because so much of the line control has to do with keeping a belly out of the line. Without that, you don't have much of a hope of tracking your fly, hooking fish, avoiding rip-tide levels of drag that snatch your fly along at jet speed, or keeping the line from getting snagged on bottom structure or looped around stream boulders.
Important: keep your leader short, or it will belly too- usually upward from the sunk line. You want the fly to stay in the same water column as the line. Between 3' and 4' is about right. Any more than 5' and it can be very tough to hook fish.
The sink-tip leaders I've used have a tendency to seriously hinge, particularly with the lighter line weights. I think it's worth it to use an actual sink-tip line. They have a tendency to hinge also, but they're more manageable than the weighted leaders I've tried.
The weighted leaders tend to have a larger diameter than the tapered tip of the floating line they're connected to, along with much more weight than a tapered line tip- at least in the line weights I was using. And using a loop to loop connection makes the hinging problem even worse. You might be able to compensate on a 7 weight by trimming the tip back to get a smoother transition and turnover, and you might be able to improve on a loop connection by using one of those plastic snap connectors, or even doing a splice. But I think it's easier just to get a sink-tip line.
(You can fish nymphs with sink-tips, too, although not with the usual nymphing techniques. Although the real fun is using them with unweighted soft hackles.)
This is where it gets complicated: sink-tip lines come with different length sinking tips. And it's tough to know the best length without having some experience with them on the water.
I recommend getting a one with a fast-sinking 15' tip in the proper line weight to start. Depending on the flies you're throwing, you may find that one size lighter or heavier works better. Line weight is less critical than with floating lines. 15' is more than you need for most rivers, but it's better than buying one that's too short.
Then cast it on the river- preferably in clear water conditions where you can see it underwater- and observe how it works on the drift and retrieve in different depths of water, and current speeds.
That will all depend on the water you're fishing, of course. That's why I said that it gets complicated. For ease of line handling and hooking fish, shorter is always better- preferably no more than the rod length plus 2'. The most important thing is to get the fly down and tracking more or less straight off the rod tip.
In a situation like the one you're describing- 6', fast current- I'm not even sure that a sink-tip is the right answer. You may find that you prefer a full sinking line- a type III or IV fast-sinker.
One seriously effective way of fishing streamers for big trout holding in stretches of heavy current with a depth of 2'-4' (or in slightly slower and deeper water) is to use a type III sinking line with big flies, cast upstream, and hustle like crazy on the hand retrieve to keep the line taut, pulling the fly along in foot-long jerks, like the down stroke of strumming a guitar.
It isn't really possible to do that with a sink-tip, because it's unlikely that the fly will get to the bottom, and you'll find that the floating part of the line has a different agenda than the sinking part in fast water. The floating portion gets pulled along the water surface much faster than the sunken tip, which has a way of screwing things up monumentally.
You can work the fly down and across in fast deep runs with a sink-tip, but the drifts are short, you cover a lot less water per cast, and the presentation isn't as realistic as the upstream method. Not that the downstream method won't take fish sometimes...
