I like how googles MCC for Adwords is set up does facebook have an "MCC" type software?
I have more than 7 years of experience in digital marketing, managed over $50,000 in ad spend on various social media networks and currently managing over 30 Social media assets for various clients.
You can add as many Funding sources to your Facebook Ad account as you want but all campaigns in your Ad account would be charged to the Primary funding source failing which the secondary and so on. The best practice when you have multiple clients is to ask your clients to setup their own Facebook Ad accounts, add their own credit cards to their own accounts and then give General user access to you to enable you to manage their account and create ads on their behalf.
Facebook is all set to launch the Facebook Business Manager which according to Facebook themselves would be a way to help businesses and agencies manage their Facebook Pages, ad accounts and payment methods in one place. You can find out more about the Facebook Business Manager here: https://www.facebook.com/help/businessmanager
If you have any further questions, I'd be happy to answer them for you here: https://clarity.fm/gs.gill/expertise/use-facebook-ads-to-meet-your-business-goals