I just did what you describe.  It was a little bumpy for me.  Win 8 will not boot in legacy.  I shrunk the NTFS partition with gparted (from a bootable CD), then installed Ubuntu on the free space using gparted (don't make my mistake - linux may not see your windows partitions and will overwrite them if you use the auto function - you have to specify which parttition manually - use the custom function).  You can then boot into ubuntu in legacy mode.  If you want to use windows just go into the boot functions and change to uefi, then restart in windows.  There may be a better way to do it (I am definitely not savvy with these things), but it is an easy temporary solution that has been working for me for a few days now.  I don't plan on using windows more than a few times a month, however.

I've still not solved it with Slackware (anyway I use rEFInd to dual boot in UEFI) but since I'm going to have a Win8 pro iso soon from my msndaa account I think I'll keep away from trouble and I'll just format everything (not with gpt partition table) and proceed as usual. For those interested in how to make a dual boot in Uefi with slackware there is a nice tutorial (www.slacky.eu/.../Dual_Boot_UEFI:_Slackware_14.0_e_Windows_7) written in Italian :) After some research anyway I found out that even if it works for you, a mixed approach may be harmful because the bootloader (grub2 I suppose for Ubuntu) usually is installed in the MBR but in Uefi there's no such thing as a MBR so you may delete accidentally other things (even if there is a Protective MBR). I'm going to have more time to spend on this issue in February so if I find a nice&clean solution for making the two OS get on well with each other in Uefi I'm going of course to describe it here :D