Hi Blake!
I see you're a fellow Hamiltonian! Nice to meet you. :D
In order to use that existing Stripe account, you'll have to disable Shopify Payments. Inside the payment gateways menu where Shopify Payments is now, Stripe should be one of the recommended gateways in that dropdown. It's the first suggestion for me!
You can use one main gateway like Stripe, then Paypal, and then any number of manual payment options.
Hope that helps!
- Alex