The Tavern

At the beginning of the game, choose a skill. You only train that skill until xl3, when you [edit - may] choose a new skill. Then you only train the new skill until xl6 and so on and so forth until xl27 at which point xp does nothing for you.

Balance however you want. The point of this species is to force the player to commit to skill investments ahead of time, offering a more severe risk of ~~sunk costs~~ than typical xp allocation does. Maybe give some bonus to magic to encourage character diversification; heavy-armor melee characters have to train fewer skills on average.

edit: When you are in the skilling menu to select a skill, it tells you what skill level you will reach after your commitment (the same way it does when choosing how to allocate pots of xp). If you choose to train something to 27, you could lose the leftover xp or be allowed to reallocate - you shouldn't train anything to 27 anyways, so the implementation of this edge case doesn't matter much to me.

Do you have to choose a different skill, or can you keep training the same skill as long as you want?

you can keep training the same skill