Do I really have to apply the powder on after using foundation? Can I just only use foundation? People recommended me to use along with mineral powder but I think there is no need.

no, it's not nessecary,
but it will keep your skin looking matte.

The mineral powder should be your foundation. That is all you need unless you use a little concealer under your eyes or something. There is a setting powder that you can buy to apply after your mineral powder but it isn't really necessary.