Like Ricki said, I don't think than Thancred meant to replace Louisoix, but he does seem somewhat protective toward Minfilia, probably due to the fact they leave the leading role to a girl he took care of for a few days when she was a child (several lines of his shows his "father like" affection for her).
Btw, Echo stories takes place 15 years before 2.0, not 20 and no Thancred is not from Limsa, all he said was that he did spent some time in Limsa when he was young with the weird ninja/pirate/wuhevah brotherhood.

Oh one more thing, none of them have shown any sign of aging besides Minfilia, Cid just grew a beard that make him look older, F'lhaminn's glasses and clothes are what makes her look older, even tho you see her in echo most of the time in 1.0, there are some scenes during the actual present time where she does not look any older, even tho 15 years separate echo and 1.0 (the only way to make the difference is the color of her boots).
It's actually the same for most characters like Momodi or Miounne, I think that people just age differently there than we do in the real world.
I can't remember well but I'm pretty sure in some other thread, someone theorized that lalafell stop physically aging after a certain age (in some cutscene you can see a lalafell as a teen during an echo and later see him as an adult in present but there is no changes besides his clothes)
But now that I start to think of it.. What if this theory is not just for lalafells but for every one in Hydealyn?
That would explain why every one looks the same age, except for Minfilia who obviously became an adult, but since she's the only one we knew as a child, there is absolutely no way for us to know.

Now it might just be me overthinking it but.. It does make sense ^^'''