Is it safe for my 15 year old daughter take a regular amount of any typically adult medicines even though she's very petit and small like an 11-12 y/o?

Depends: Many over the counter meds are sold in half their prescription only strength once they are released for OTC use. Their safety profile is generally well tolerated by anyone from age 12 on. Many prescription meds/antibiotics/etc require modification for body size, and this is well known to issuing prescriber. Any product sold as a medication (not a supplement) will have appropriate warning labels.

Mgt: The pediatric dosing is usually based on weight for most medications. There are some medications dosed by age. It would be helpful to know her medical history and medication list. An inbox consultation may be booked online.