Are we stuck with the grand company once we choose one?

[Ventus]

That's exactly what it says.

You get one (possibly more?) recruit quest per company, during which you can "try them out" and see which one you prefer.

The more advanced quests will ask you to pledge to one of them and that's pretty much what they've been saying since the start of the whole Grand Company tease.

Well if things continue the way they are, the ul'dah grand company will have 90% of the people in it on every server.