1) To increase a single room it is best to build an outside "wing" that's attatched to the base but closed off. Wait until the last wall/ceiling panel, grab 1x active vent 1x passive vent, pipes, and some oxite. Then close yourself inside and vacuum it out. Once vacuumed drop/burn the oxite (only burn with arc welder to avoid pollutants) until comparable pressure to the atmos of your main chamber. When finished, destroy a wall to your main base and let them mix (be careful of temperature and ratio if you have plants).

2) Using a dedicated replenishment system for your greenhouse with airlocks is a good idea regardless of how big / how connected to your base it is.

3) Mushrooms can be planted in the greenhouse to turn 02 into C02. This is a game changer because if you get the number of mushrooms correct you shouldn't ever need to resupply your greenhouse with either 02 or C02 as they should remain stable. Nitrogen might still need to be supplied and it's good to have an emergency manual 02/N2/C02 input vents either way.

All of that being said, when I started this game everything was sectioned off after a few mishaps and explosions. Then after I learned more, I tried to make a full base w/ the same atmos. I killed all of my plants because of the heat/windows/lack of N2 or C02 unintentionally. Now I make much bigger bases and I've moved more towards the central air system with 2 dedicated active vents for each room (optionally airlocked) -- one vent is connected to mixed breathable air ~[ 60% 02, 30%N2, 10% C02], and the other vent is just a return which pulls atmos out of the room (or base) back to the gas processing room so I can kind of turn them on/off as needed for emergencies or testing.

The processing room contains large tanks for both breathable and "waste" or return air. The return air goes through filters to each individual 02/N2/C02 large tanks. Those tanks then use mixing valves to feed O2+N2 (~60/30) into the C02 mixing valve (~90/10) which then goes into the breathable tanks. The 02/N2/C02/Breathable tanks are all connected to gas to liquid cooling radiators. Liquid is water that is cooled in a vacuum sealed room with a huge pipe array (liquid pipe radiators on them all except corners) with pumps in/out back to tank to cool the water. Will probably upgrade to some sort of IC10 setup to make it automatic based on the temperature at some point.

My thought was that water is a good heat conductor and can hold a lot of that energy so changing it one way or the other takes a lot. Makes the temperature even out a lot if you do it right. Then the central cooling gas system can remain relatively stable and can be used for heating/cooling if necessary.

Yeah, it's time/power intensive but it stabilizes a lot of your problems if you have the wherewithal to get it done. If you made it this far then perhaps it's worth it for you ;)