Do crewmembers ever "level up"? And how to make friends?

What I can see it some kind of very gradual re-balancing, but only on skills that are maxed otherwise the characters would lose their individuality.
For example a skill three miner could drop to two through lack of use, but not below that, as they are a good miner. On the other hand a character who has a one in medical and never uses it could drop to zero giving them a 'free' skill slot to maybe experiment with mining.

Crew would maintain a varied skill distribution if there is a maximum level such that nobody can max out every skill.
Gaining skill in fields through practice runs into the problem that a skill 0 miner is incapable of Mining.  They will never be able to try out mining.  On the other hand, Weapons 0 people can still fire guns.

My wish is to buy slaves from slavers.  They would cost credits proportional to how many skills they have.  A cheap slave could become more capable over time by gaining levels, or buy the skills of an expensive slave right away.  They could have a rebellion or higher surrender chance, so you have to keep control of them.  And obviously, the option to release the slave or, after earning sufficient experience, recruit as a full crew member.