Hi matt,
A lot of questions here or at least, the questions have loads of answers!!
When I go trout fishing its normally roughly 6ft of one leader joined to another 6ft of a lighter leader material with a fly on the dropper and a fly on the point
For dry fly fishing my standard leader is a 12' tapered job with either 4 or 5x at the end depending upon what I am throwing. I loop the tapered leader and and add about 2' of tippet, this is a starting length. If on a very flat glide I will go up to 16' if required and if prospecting with a big klink in fast stuff I may be using a 7.5' tapered leader made up to about 10' with tippet. I never tend to fish two dries on a river.
Leader length will be reduced and maybe beefed up in a wind. This is based on fishing middle & lower river, if fishing on small streams / headwaters then I would reduce leader length to something more manageable. Best guide really is to use the longest leader that you can control in any given situation / condition.
Also is sinkant a requirement when using weighted flies?
I always degrease my tippet if only to take the shine off it but usually just the last third, if fishing nymphs I tend to use straight mono as it sinks faster than a bulky tapered leader, the weight of the nymph usually is sufficient to turn the leader. I never degrease / try to sink the tapered section of the leader.
When fishing spiders / wets I will use a 6' tapered leader down to 4X the usually make up the rest of the leader with 5x, three flies spaced over 7 or 8 feet to give a total length of 13 - 14'. in this case I wil l apply floatant to the tapered section of the leader, a bit like buzzer fishing you are watching for takes and the end of the line / leader butt is your indicator.
Matt, these probably looked like straightforward questions but there are so many exceptions. Best bet is get the last 12 / 24 months issues of T&S out and read all of Paul Procters articles!
Cheers
Ben