It doesn't need to be tapered, I would run something like 6' straight flouro. It needs to be long enough that the fish can not see your floating tip but not too long that turning over the fly becomes difficult when casting. I rarely use floating tips, but when I do I use a clear floating poly leader and only about 3-4ft of leader, because the poly is clear it doesn't necessetate a long leader.
That would depend on the size fly I was using, the environment and the style of fishing I was doing.
For example the only place I might use a floating tip is in a shallow 2-3' riffle
- casting a streamer it might be 4'-5' of straight 3x for trout on up to 0x for larger fish, larger flies.
- casting some soft hackle wet flys 7.5'- 9' of tapered mono to 4x for trout.
You also have to determine how long a leader YOU can cast effectively. Like Ard said in a post once, spey casting is like being Goldilocks your line setup must be juuuust right.
As others have pointed out, a Skagit head is better suited to sink tips and heavy flies, but you can make a Skagit work with floaters and dries. It won't be pretty tho'. If you want to fish a floating line or dries, a Scandi or "hybrid" head like the Airflo Rage is a better choice.
I use 10' poly or versileaders most of the time these days. If I'm swinging sink tips I'll use a 2'-3' tippet of straight 15# Maxima Chameleon looped to the sink tip; if I'm skating dries I'll use a floating poly/versileader and a 9'-ish tippet of straight 12# Maxima Ultragreen. This would be fishing for salmon or steelhead on a 13'-ish 8 weight rod.
I build tapered leaders when using a floating tip on my skagit or commando heads only if I am casting large surface flies like Dhalburg divers, poppers or large surface pike/muskie surface streamers and it purely just to get it to turn over and for toothy fish I have some intruder wire by the fly.