I too have asked about collars
I was scared about using one on her .. At a poster's recommendation I did buy a thin/small break away cat collar for her and it seems to be working out fine. I leave it on her for a couple hours during the day to get her use to it ( I never leave it on at night or if she is in her pen) I wanted a collar because I switch harness' alot and hated tring to switch the tags but a poster ( sorry can't remember who) told me to get etra lobster claw type clips so I could easily switch the tags from one harness to another , which is working great. I still put the collar on her throughout the day though so she can get use to wearing one if she should need to ( plus her charms look cute
)