Do the other filters also have UV filtering in them, or is it something else that prevents using both? I’m thinking if I need a UV filter I’d need it regardless of whether I also needed for instance a grey grad. Sorry for the questions, just trying to sort the logic in my head.
Either a Skylight 1B or UV(0) (designations vary among manufacturers) can be left in place to afford some protection of the front element of the lens. As you have seen, when using E6 films a presence of mind is beneficial to judge the effect that a Skylight 1B filter, particularly, will have on the image at a certain time of the day.
Stacking filters is not good practice for any lens, and will progressively degrade the image quality, in addition to greatly increasing the risk of intra-surface ghosting/flare and condensation.
Contrary to populist opinion, a filter here does not have any humanly-discernible effect on imaging, nor for that matter, a gelatin filter in a specific in-situ holder at the rear element. A few points to consider. In the case of a front element being ED glass, no filter should then be placed there. An aspherical front element is sometimes protected by its own front protection glass (as with many Canon L-series lenses). Many still have such a protruding rounded profile, so that attaching a filter can actually cause abrasion damage. The third point is that any filter must be matched to the quality of the lens in use; it is pointless to buy a $5,000 bells-and-whistles lens and slap on a $10 bottletop Chinese filter (some of my own filters cost more than the camera!). The risk of ghosting and flare remains, (and always will exponentially greater with square/rectangular slip-in filters) but today's glass filters are doing all that is possible to greatly reduce, if not eliminate, both things. The rest is up to where the photographer aims his camera if there is sun in the view!