hmm to be honest, I'd be worried too. Is there a reason you want to have it done at 6 months? I'm sure everything will be ok, but you are right, she's so tiny. What did your Vet say? If it was me, I guess I would wait till she was 8 months to a Year, but your Vet should know best. Sorry I have been No help.
Tia is just so cute, I'd even worry about her, and we have never met.
Please keep us posted.