Question regarding Mountain Lion, Parallels and Windows

I have a 2011 MBP successfully running Parallels 7 and Windows 7. I have finally convinced my husband to make the jump from PC to Mac and just ordered him a 13" MBA 256GB with 8MB RAM. I am going to order the Parallels bundle as soon as it arrives and was going to get on Amazon and order a single user copy of Windows 7. My question is: if he upgrades to Mountain Lion when it is released, will Mountain Lion be compatible with Windows 7 or will we have to purchase another copy of Windows (8)?

Okay, one more question. I mentioned updating to Windows 8 later and he said he'd want to stay with Windows 7 as some of the company software he uses has just now been brought up to Windows 7 compatibility and who knows when it will become compatible with 8. Should I buy him the 32-bit or 64-bit copy of Windows 7 (I read in Amazon reviews that 32-bit is more backwards compatible)?