Length and ID of the beer lines?
Serving pressure and carb pressure are identical if you use the two week 'set and forget' method. Use
this chart to find the pressure you need to set to at the temperature the beer is at.
For beer hose length, rule of thumb is 1' of 3/16" ID tubing (Bevlex) for each pound of pressure the keg/beer is at. You can fudge it a little bit, but at 13psi, you should have ~12 feet of beer line going from the keg to tap/faucet.
Are you using picnic taps or actual faucets (tower, through the fridge door, etc.)?? I quickly learned to hate the picnic taps and installed Perlick faucets through the door of my brew fridge. IMO, much better since you don't need to open the fridge door in order to pull a pint.
Also, IS the beer overcarbonated or not? If not, then it's not a point where you need to reduce carbonation, but increase beer hose length. IF it's overcarbonated too, pull the gas QD from the keg(s) and vent it 2-4 times a day for about 2-3 days before connecting the gas line to it again.
BTW, I have my CO2 tank outside the brew fridge so that I can fit more kegs inside. But I have four 3 gallon kegs in my brew fridge and a 20# CO2 tank connected to a dual body regulator. With a pair of manifolds inside the fridge, I can feed two different pressures into the fridge and have different brews carbonated to different levels and served properly.