Instead of buying a sink-tip line you might think about using leadheads.
I think Orvi$ might sell some but the ones I use I made myself from Cortland LC-13 (lead core line, 13 grains per foot). Cortland might even sell a DIY kit that includes some of those 'chinese finger puzzle' loops...
I carry a 6", a 1", and a 2" section in my trout kit so that I can use any combination from 6" to 3 1/2 feet on my floating lines
In my salt water kit I carry sections of 2 and 4 feet.
I've even used these on full sinking lines for extra depth
You can loop these either between the line and the leader, or between the leader and the tippet depending on what you need to accomplish.
They work well and you can easily adjust the length you need.
The casting is not pretty though
If you go the sink tip route do you just have a separate reel or spool loaded with it so you can switch out? Finally does the recommendation change if it is a river/stream vs. a lake/pond?
As a somewhat general rule, sinktips are for moving water, fullsink in still water
Most people carry extra lines on spare spools. I find it easier to switch out reels myself
It's possible, but really more trouble than it's worth, to just switch lines if you use a loop2loop connection between the backing and the line.
You'll need your backing loop to be large enough to pass your whole line through in order to do this
Like I said....it's possible do this .....but it's a major pain in the @$$