i'm curious if my 120mm radiator will be able to cool it effectively
Your question is will it cool them effectively. In my opinion, no. Will it cool them. Yes. A lot has to do with what kind of shroud/fans you put on it. If you use a 150cfm Delta that sounds like a freight train, then maybe yes. If you want decent cooling with reasonable noise, then go spend $25 and mod a dual heatercore and make a shroud of some kind for it.
Yes, a dual heatercore can be mounted to a swiftech radbox. I have seen several systems here on OC forums that have done exactly that. There are many different ways to mount a heatercore. I built a shroud box had mine mounted sideways on the back.
I sold this shroud/dual heatercore, but I still have my BIXIII mounted the same way.
You can use two rads, one mounted in the rear and one mounted in front, but that always adds extra tubing and connectors which can make for a mess and can reduce efficiency. It is easier to use one rad in one place.