I’m trying to get 6 different crew members on the ship but my crew is full; should I kill a current crew member to ensure space to take on new crew, or will I be given the option to swap if a new potential crew member offers their services?
Yes, you will be able to gain new crew members. When you get a crew from an event, e.g., an interface will appear at the end of the event showing you every new crew member + the new guy (who will helpfully be flagged as “new”). You then have to chose to dismiss a member since you are over the limit. If you see crew for sale in a store, you can go into your ship screen and dismiss a member there so that you can buy a new one.
In the comments, ChargingPun confirms that when you are on the “crew dismissal” screen, you can still mouse over crew members to see their skills.
Also please note that as of patch 1.5.4/the recent add on, potential crew you encounter can come with skills which have gained experience. It is possible to get a crew with max [insert skill]. This is tremendously useful, as it both allows one to more easily make up for later-sector mistakes as well as maybe find someone with decent shields skill (heh).