NOTATIO

On small small cue staves in a vocal score should one always use piano clefs? In the full score the trombone is in the tenor clef but I've used the treble on the cue stave, which looks a bit odd to me, but I suspect is the best practice (after all one doesn't use transpositions on such staves).

The tempo is = 80

Yes, that looks good. It is because the music is going to be played on the piano or similar keyboard instrument, to help singers.
Consequently, all should be non-transposed and in the "regular" (G/F) clefs.

Don't have score examples on hand but that's what I've seen in the past in all such cases.

A. Arnstein felt that one should not use a non-native clef for any cue. Gould seems to allow treble and bass clefs for all instrument but not non-native C-clefs.

Thank you all for confirming what I'd suspected was the case.