Marriage between a mangalik and non manglik is possible, provided other things in kundli milan are strong. However, regarding nadi dosha, you need to check if you have Nadi paad vedha or not, if no vedha then Nadi dosha gets cancelled, which is good for your relationship. As you say that your Guna milan score is 15 out of 36. It is best to do Graha milan (Matching of all planets) along with checking the 7th house of marriage. If your Graha milan is good and Nadi dosha gets cancelled, then marriage can be considered with out any issues.
Blessings,
Navneet Khanna
Astrologer