Re TV 31mm Nagler in GSO 12" f5 - should be fine, but note you will need to cope with the weight. I don't have the eyepiece, but the previous owner of the GSO 12" f5 I bought used it very happily.
Suggest you search the
www.cloudynights.com forum - look in the reviews and the eyepieces forum - they is plenty of discussion of the 31mm Nagler - often in comparison to other eyepieces.