As I understand it, and if I'm wrong, feel free to correct me, but as I understood, AVG (the free version) is not a "realtime" AV. Meaning that you have to scan for viruses after they're embedded into your OS. I've never had, nor do I want, the paid version. The one install that I did have (the free one) was preinstalled on a notebook that I purchased on eBay last year. I promptly removed it and replaced it with Avast (the free one), it does offer "realtime" protection. And prior to MSE being released, it was my default AV for all of my Windows OS's.
As long as it is a standalone scanner, you can use whatever you wish with MSE. I do, I use Malwarebytes & SAS.
With XP, I could understand running two, I did (but again, I don't advise it), but Windows 7 is quite different from XP. The security is much better, and MSE will meet your needs without creating a burden on the system. I do recommend a backup standalone scanner, such as Malwarebytes, all you need to do is update it, then do a short or long scan. The same with SAS, update it & scan. It finds a lot of adware & tracking cookies, and I run it on a monthly basis.
I hope that you do find Windows 7 a easy OS to keep secure. As long as you install MSE, set it to auto update & do a short scan daily, you should be fine. A full scan weekly is good, too. If you want to add the free version of AVG to do manual scans with, I suppose it would be OK, if you don't have problems with it.
Cat