I am running windows 8 though bootcamp on my mac (10.8). My question is if a program (specifically Borderlands 2) says it runs on windows 7 will it work in windows 8
It should work but there's no way to guarantee that. In each new release of Windows, Microsoft has been very careful, going all the way back to the original Win NT 3.1 Beta release in July 1992, to maintain compatibility for any existing Win32 APIs. If they need to make changes that might change the signature (the arguments or the return type) of an API to add functionality, they've consistently chosen to add new APIs, usually with an Ex suffix, rather than change the existing APIs. This allows each new release of Windows to run most existing apps with no problem.
Where this breaks down is when Microsoft is forced to make a bigger architectural change. For example, Vista introduced the secure desktop and User Account Control. This broke certain applications, e.g., any existing
Microsoft added stuff in Win8 but it appears that consistent with past practice that they've been careful to avoid changing stuff that's there. The big architectural change is the Metro UI but while I could certainly be proved wrong, I doubt that will break most apps.
But applications depend on more than just the API to run properly. They often contain assumptions the developer may not have even been aware might not be safe. For example, he may have assumed that Microsoft would always put certain files or registry keys or who-knows-what in certain places.
So I guess I encourage you to be optimistic and see what happens. As Reagan put it, "Trust but verify."
You might want to manually configure the compatibility mode for that program. To do so, right click your .exe and select Properties, click on Compatibility tab, find Compatibility Mode section, check RUn this program in compatibility mode for, select Windows 7 (Drop down menu will be enabled once you tick the checkbox)