At level 20 you will have been able to use 20 ability credits on your character. There are only 11 medic abilities. You would then have 9 abilities to purchase from either the universal pool or other classes to increase your options when using those classes with this character.
You can focus on one class for a character, or
you can make 2 well rounded but not quite complete classes for one character, or
you can follow a jack-of-all-trades type approach where you pick and choose abilities so each class has something and you have some of the important universal abilities.
You keep one character during a match (no switching characters or body types) but you can change classes.
In between matches you can go back to the main screen and choose a new character or a new body type for the character you are using.