An authority on Thai cuisine might be better, but Thai basil has a distinct taste, almost a cross
between mint and basil (which I've seen as a substitute since it's hard to find). I've substituted the really tiny chilies from the grocery store for Thai chilies. They are super small and are green and/or red. Maybe they are serrano?? This recipe only calls for 1 Tbsp. so it won't make that big of a difference. Maybe you could use a basil and a mint leaf for the Thai basil.