Welcome to the yard Mick.
I figured I'd throw in my 2 cents on the sheaths . . .
"Sandwich" style kydex are nice, but I wouldn't say they are any slimmer than a folded "taco" style kydex, nor would I say that one is tighter than the other. I have 3 folded kydex that lock up tight. There may be better mounting options with more rivets but as long as it supports a tek-lok, you should be okay.
I would think that the S5 might be a little big for inside the pants carry but each person wears theirs differently.
I do not own a mashed cat or okuden but many people pair them with their Scarpyard blades. I have BuyBrown and Rainwalker and am very pleased with both. I wouldn't count on an offering directly from Scrapyard and for the S5LE, it might be worth it to pay for a custom one. Good luck whatever you choose.