A 16g is a perfect home for a betta just by its self in my opinion but I dont think
5-6 Corey's would bother him too much..
The problem with adding mid level swimmers with the betta mostly comes down to tank size and what kind of fish you keep with him.
You don't want your fish to ever be stressed out.
I'm not a fish exspert. I can only give you advice based on my 20g aquarium.
My betta seems stressed over the 12 peaceful tetras I'm keeping with him..
I want to get a 100 gal
or at the least a 55..
My goal is to give my betta the option to not have to constantly be around other fish 24/7 I hope I've helpt in any way.