You can run arbitrary java versions besides each other. There is no need to have Mendix run on Java 7 as well. Mendix will probably run on Java 7, but is not supported (and Java 7 have several known issues)
You can install JDK 6 and 7 next to each other. Actually, the installer will install JDK 6 update 27 if you only have JDK 7.