10-20 is actually design for crop sensor so you will have strong distortion at the edges sides specially if you shooting at 10mm. You using A7/r?
I'm not familiar with 10-20 but if you shoot straight dead horizon you should be able to have a straight horizon that will help when stitching at least you have something easy to align when stitching but if you shooting slightly tilted up or down you may be able to stitch but don't expect tall pano you may ended up narrow pano and you may need more shots to stitch and the overlap needs to be big.
I'm using the sel1018 with a7 I'm FF mode. Never used the 10-20 before. Hmm. I use the panorama function and hugin to stitch my panos. I suppose you are doing manual stitching?