should I render out the 6 images for cubic panorama or what is the best way for this sort of thing?
I think this is the best way, as cubic panoramas feature the best viewing quality and the panotools use them anyway. Make sure you name them by this scheme, so the tools know how to align the cubefaces:
"pano_l" - left
"pano_f" - front
"pano_r" - right
"pano_b" - back
"pano_u" - up
"pano_d" - down
I already tried this by myself, following the kmakemultires tutorial (is that what I should be using?)
If you want. :) It depends on what you want to do, but it's no mistake. Keep on practicing with it.
The sides of the cube all have separate colors and the camera is set to have a fov of 90 degrees (correct?).
Use a (much) lower fov value and try some fisheye settings. That should fix it.