You can play a player at any position. Their performance at that position will be based on the ratings they have in their skills, compared to which skills are important for that position.
That being said, a player's recommended position is based on which position's salary formula yields the highest salary for the player. So a player with a recommended position of Point Guard is pretty likely to help your team more as a Point Guard/Shooting Guard than as a Center.
Training is based on which position the player received minutes at during the week. If you train a player at PF/C based skills and he gains a significant amount of skill increases at those skills, his recommended position may change at the next salary update, at the end of a season.