Hi guys.
I would go with the suggested sizing, if I were you. I am about 5' 8", and normally small does me, so I opted for small! In hindsight, medium might have been a better fit - but i am enjoying the 'snug' fit all the same!
Seams are not flat-lock. They are just regular seams (?!). Like I said before, they don't get in the way, nor do they feel uncomfortable, but then again this is my first skinsuit, so not really got anything to compare it to. You guys might be more used to a better quality suit, but for a first timer i'd definitely say it was worth the £35 I paid.
Might try out other suppliers in time... :)