Custom Payment Integration

I have been looking for how to integrate custom payment gateway into prosites, membership pro and marketpress. How do i go about it. I already have my API from the payment company ( but can’t seem to know how. Please can someone give me a lead. I am a php developer and have integrated custom payment integration into woocommerce before. Please support guide me to the required resources.

Secondly, I don’t like the css of the prosite plan table how do i change it to look better positioned like that of wpmudev.