You know you can have 2 versions of Java at the same time, just select an alternate location on the installer (never done this myself but I would expect it will work). That way those who need Java 8 can do so while also having Java 7 for Minecraft.
I don’t think it would be a good idea setting the minimum Java version to 8. Minecraft officially supports from 6 up, so does Forge and so should Sponge.
I wouldn’t want Sponge to be more limited than Vanilla in terms of hardware/software requirements.