Medical Treatment is useful for gaining experience only if you take advantage of not being able to die due to damage. You stand in front of a creature/bandit while you are by yourself, attack, your health is reduced to 1 and you heal. Your attack and healing timers will finish at almost the same time so you can repeat. Once you can die due to damage then it will no longer be possible. There are other things Medical Treatment is used for as well.
Management is for all production, building facilities, mining costs/yield and government tax rates. It is one of the more useful skills in the Combine if you wish to produce anything.
Perception/Stealth counteract each other. Perception allows you to see further on your own and to detect stealthed enemies. Stealth allows you to move undetected. These are useful support skills but they are essentially tactical options.
Diplomacy/Trading is not very useful. If you have a high skill you gain reduced costs of hiring npc's and gain more credits from facility income. It is an iffy skill in the current state of the Combine but hopefully it will be more useful in the future.
No, you cannot create a lock on a room to make it yours. The purpose of doors/locks is to protect assets within a ship or facility from others that may have access to the facility. Of course, with an electronic lock breaker and the Crafting/Slicing skill you can bypass locks and doors.
Useful skills for a freelancer
Computer Operations - Prospecting is lucrative as many factions do not have active people with a high skill. They are often willing to pay others fairly high fees because of the skill requirement but also because it can be boring in the long run. 6 hour timers with a skill rank of 5.
Crafting/Slicing - People are often interested in doors/locks to be placed on their ships and are willing to pay high fees. Of course, you have to safe guard yourself by never boarding a ship that is owned by someone you do not trust. Once again this is a time consuming process and can be boring. Timers are as long as or longer than prospecting timers.
Hunting (Combat Skills - Mostly Dexterity) - You can gain a lot of experience and if you are lucky you can get drops that you can sell for high prices. This is what I did for the first few months and it can be lucrative. Time consuming but not as boring as the other options. 30 minute timers.
There are other useful ways of gaining credits as a freelancer but these are the most dependable source if you are not interested in spending a lot of time buying/selling.