If you want to practice casting with sinking lines, you need not worry about curve casts, slack line casts, or recoil casting. The faster the sinking lines the smaller diameter they are. I think thats why a lot of people use a 5w rod and 6w line etc. etc. It could also be a hold over from years ago with the 5/6 etc rating.If you are doing this to better your stillwater casting, dh'ing and distance are what you're shooting for. Accuracy would also be included in this game. Grab a box sit on it like you're in a tube or kickboat and start casting. Video yourself.
A 9', 9'6" or 10' fast rod will be your friend.
You won't pick up line off the water with a sinker like you will with a floater, you already mentioned that.
I've marked all my lines at 30'& 60'. Used Loons UV Knot Sense. Mark with a Sharpie and dab a little Knot Sense on it. One dab for 30 2 dabs at 60'. You can feel it in your fingers when it goes through on the cast and when your retrieving it. It won't get caught in the guides either. It's just to determine how much line you have out. With a little practice and a good rod you should be able to cast 60'+ in a little while. Strip out a little more and kick backwards.
If your practicing for river fishing- then nevermind.

RTJ