Just got a couple of  questions to ask...

1) Is it possible for a o-ring to swell if you put TOO much oil?

Not if it is the correct "O" ring and you are using the correct oil...what oil are you using?

2) I need some o-rings, does it really matter what kind i get? or are they all the same?

There are several types of elastomer used to make "O" rings. You'll need nothing fancy for the marker. Dozens of on-line stores and your local paintball dealer have what you need.

3) my Custom pro (regular no r/t or egrip) is double firing. I found that the rear bolt o-ring was REALLY loose. could replacing the o-ring solve my issue? i checked and my sear and rear bolt aren't rounded off.

Most likely replacing the "O" ring will fix the problem. But, what do you mean by "double firing?" Two balls come out when you pull the trigger or the marker fires twice then returns to cocked position?

Two balls coming out from one shot is probably a worn out detent. The marker firing twice when you pull the trigger once can be caused by a couple of things. If after firing that second ball the marker has to re-cocked by hand then that indicates that the hammer (rear bolt) is not being blown back far enough for the sear to engage. A new "O" ring with some fresh lube will likely fix it.

Sometimes when the sear or hammer notch is worn things will not engage to hold the hammer in cock position. The hammer travels part way back and either doesn't engage the sear or a faulty "o" ring did not provide enough of a seal so that the gas could act with full force against the hammer to blow it to the full rearward position where the sear would engage. If you are talking about worn and loose "O" rings I'd say that is likely the problem.

4) is there a way to shrink an o-ring that swelled?

Yes, leave it out in the (warm) air to allow the solvents that have effected it to evaporate out of it. Still, if it does return to size it may have lost some of its elasticity and though it fits it won't seal correctly.