I can testify that Nod32 V. 2.7 works flawlessly with the firewall. I haven’t tested 3.0 of Nod 32 but I know it uses a proxy and that creates issues with the firewall logging and most likely informing the user of malware, but I believe some users have found workarounds.
[Edit] Ok, never mind what I said… free… hmmm. In case you still are interested in Nod they do have Student Licenses running around somewhere I think. Those tend to be relatively cheap. Anyway, to the free ones.
I’ve used AVG and Avast. I much prefer the layout of AVG and it has always worked well whenever I needed it. For me, the AVAST look just wasn’t doing it and configuration wasn’t as simple, IMO. Also, at least for me AVG also ran lighter than AVAST. It’s probably not the same on all, but it was on mine. I BELIEVE some options on AVG free are limited (since it is free) like scheduling a specific time for a scan or maybe configuring updates if memory serves me right. I’m not sure of these limitations on AVAST because I could never use it long enough to figure it out (it always ■■■■■■ me off before then).
As far as detection rates go, http://www.av-comparatives.org/, should help you if you care to look at them. I won’t bother to go into it because that’ll just start a war here. Personally, I think AVG’s detection rates are good enough. (A note about A-V Comparatives, I believe the signatures it lists for both AVG and AVAST are for the paid version, but I see no reason why they shouldn’t be exactly the same for the free version).
My father uses antivir free fine with CFP. Though I don’t have much experience with it, I don’t believe it has automatic updates in the free version meaning one would have to remember to press the update button unless it reminds you. He likes it just fine, no muss no fuss on his system.
[YET ANOTHER EDIT] Lol, Sded, you reply too fast while I am still typing. Feel free to disregard what I said if you have already chosen, but I spent so long typing I’m gonna post it! 
Glad you have chosen. Also, to answer your question I would still use antivirus in real time just in case CFP misses something (or you make the mistake of allowing it through). It will save you from having to do scans online weekly unless you want to. I’m in a University, and frankly when school gets going online scans delay me un necessarily.
Dave