I meant I wanted to shoot the whole sky with the comet in it, and feature part of the terrestrial view as well.
I see, well, then perhaps a fish-eye would serve you good. But generally, if you choose a very short focal length, the comet will not look as impressive (unless it is
very impressive). If you want terrestrial features in the foreground, another strategy is to choose a time when the comet is nearer the horizon, and use a focal length more appropriate for the comet.
Incidentally, how well would a 300mm lens work on the Vixen Polarie?
I have no personal experience with it. The Polarie is intended for more wide-angle work, but is rated for 7 Ibs. of load, so it could potentially take a 300/4 for some 30s, if properly polar aligned. A 300/2.8 would probably be too much, including mount head and camera. An Astro-Trac
could be the better mobile option, or a dedicated "light" tracking mount like the iOptron ZEQ25GT
if mobility was not essential (i.e. no hiking with equipment unless you have plenty of assistents).
At 300mm it would be difficult to get foreground (but could yield spectacular results if you did; compare with full moon pictures showing foreground). You would ideally have the foreground high above you (a mountain or hill) to avoid seeing the comet through too much atmosphere. For a foreground project, I think 35-100mm would be a better trade-off between seeing both the comet and the foreground well enough. But as you say, it depends a lot on the comet.
If you haven't already, I would recommend you to look at other's comet pictures from past great comets, to probe the possibilities and get inspiration. Just remember that the last really great comet, comet Hale-Bopp, was in the pre-digital era (1997), so options were more limited then.