Yeah def BN would be ok. Depending on which types of polys any synodontis would be good, some synos get big some stay real small. Obviously if it will fit in poly mouth it may get eaten so just go with appropriate size for staying out of poly mouth.
The only issues is with carnivorous/omnivorous plecos, they may go after your polys. They like slime coats and also may try to actually eat parts of your poly. Just watch. Seems common are some of the worse offenders. I had a common kill an Oscar, I was young and didn't realize what was happening until to late. My lfs at the time had no idea that they would do that, they also told me the little endi I purchased wouldn't get more the 8" and was a community fish........ live and learn.
with that crew id worry the endli or ornate will eat anyone before the plecos get a chance. I'm just going from my only time of owning an endli he ate everything in the tank eventually. Any thing I tried to add was food to him.