Will using a thong bra in recovery prevent symmastia for patients with mild pectus excavatum?

A bra is not likely to have any relevance regarding symmastia. Experienced surgeons are careful not to dissect the pockets too closely so as to avoid this complication, which is better prevented than treated. This is particularly true for patients with narrow chests, like yours. Best to discuss your concerns with your surgeon.

Dear ela1330,
the best prevention of symmastia is good implant choice and meticulous surgical technique. If you are considering surgery, I would suggest you consult a board-certified plastic surgeon. Only after a thorough examination, you will get more information and recommendations.
Daniel Barrett, MDCertified, American Board of Plastic SurgeryMember, American Society of Plastic SurgeryMember, American Society of Aesthetic Plastic Surgery

I have not had issues with symmastia in those with pectus nor do I use thong bras. However, every surgeon has his/her own opinions about bras.