Hey. I looked into flight suits before since my babies go everywhere with me. I was thinking of having them flighted instead BUT from what I have read and heard none are really good for tiels. Most are too big and uncomfortable for a little tiel. Maybe you'll have better luck- I just don't trust a little string with a flighted tiel..
I don't trust the velcro on them to use them on fully flighted birds. I have one, but it's a bit big for Cookie, and Bailee is even smaller than her. She really really hates wearing it, and it's a bit of a pain in the butt to put on. Then i'm nervous when she's outside in it because i think the velcro won't hold. I stopped using it ages ago, hehe.
A better harness that's supposed to be really good for small birds like tiels is the Aviator Harness. If they weren't so expensive here i wouldn't mind trying one out. They're supposed to be more comfortable and less restrictive.