Will having multiple gateways active at the same time cause any problems? Like PayPal and Stripe?
No but you cannot have multiple pro forms on a single page. Paypal buttons and Stripe pro-from on one page is okay. Two pro-forms is not okay (quite annoying limitation - as well as the reload when you chose another field on the dropdown menu for checkout choice)
Great. We created our own custom forms so users can buy a membership and make other purchases all at once. So it might work!