New kegger needs help

I just started kegging. I bought a used kegerator that has an external CO2 tank, one primary regulator, and a gas hose through the door that connects to a 4-way gas splitter. I have two questions about my setup that I could use some help with.

First, what happens if I have 2 different beers, brewed at different times, in separate kegs? For example, say I brew an IPA on day 1 and then a vienna lager on day 20. Since I plan to carbonate by setting a higher pressure, e.g. 30 psi, for a number of hours, before decreasing to serving pressure, what would I do when the vienna lager is ready to carbonate? The regulator would then be set at 8 - 10 psi to serve the IPA. Would I disconnect the IPA until the lager is ready to be decreased to serving pressure? Obviously, I would not be able to serve the IPA during that time.

Second, having the gas hose come through the kegerator door is pretty inconvenient, since it moves every time the door is opened and closed. I would prefer to bring it in through a side, or even the back. How would I know where it is safe to drill a hole into my kegerator? How do you actually do it? Also, how would I close the old hold in the door?

Without secondary regulators you can either carbonate at serving pressure or disconnect the CO2 from the already carbonated beer to quick carb the uncarbonated one. Unless the already carbonated keg is full, or nearly full. You can pull a few pints without the gas connected. The fuller the keg, the fewer pints. When the pour is noticeably slower, disconnect the keg you're carbing, turn down the regulator and hit the keg you're serving from with a couple more lbs. than your serving pressure.

I use 12psi as serving pressure. Almost all of my beers are APAs, IPAs or lagers, so around the same range for carbonation levels. Doing this allows me to fully carbonate a keg in about 12 to 14 day without changing the regulator setting. You do need to have long enough beer lines to balance at higher pressurre. I usually have 2 beers on tap and one carbonation/conditioning, so this time frame works well for me.

Does your splitter have valves? How many taps does the kegerator have? I use one tank/regulator for 3-4 taps. Just shut off the keg at the splitter and carbonate the other. 

If you want to carbonate quickly set it to 35-40 psi and roll the keg like a baby in your lap for a minute. Then check carbonation.

Just use the set it and forget it method for carbonating. It is by far the simplest and foolproof method I know. Hook up your keg with the CO2 the regulator set to serving pressure - let it sit for 7 to 10 days - done. You might not have beer tomorrow but your beer will benefit from the extra time spent conditioning.