avr |
The spell squeeze doesn't reduce size but does let you squeeze without penalty. Similarly there's a few magic items which give the compression ability IIRC.
Pup shape and reduce animal work for animals.
A dwindling bullet might work if the target deliberately fails a save, while it references reduce person it doesn't explicitly say you can't use it on other types. Only 1d4 rounds duration though.
relativemass |
You could just make up a Reduce Monster spell that your back guys have invented. Sorry, I don't have a good by-the-book solution to your problem.
You could use big snakes, they could reasonably fit through narrow passages. Earth elementals have Earth Glide, so they could be partly in an earth/stone wall while fighting in a narrow passage.