DBPro can be used to edit the BCD & boot manager settings.
Never tried on a preinstalled W8 OEM laptop.
Normally its recommended to install the newest OS after the older, not the older after the newest.
It should work fine I would think, you may have to tweak your BIOS first though to get W7 to run.
Using the W8 Disk Mgmt tools to shrink & create a new partition (thats how I do mine).