Thank you for reaching out today with your question. The short answer is yes, this should be possible.
If you have PayFast enabled on your store, this will be your primary gateway that is used to process credit card payments. You can then activate PayPal Express Checkout to give customers the option to pay via PayPal rather than with their credit card.
Both Splitit and PUT IT ON LAY-BUY are listed as alternative payment methods, which can be enabled alongside a primary gateway such as PayFast. Once you have merchant accounts with these gateways, you'll be able to connect your accounts with them to your Shopify store and use their platform to process payments from customers.
I hope this explains things but let me know if you have any further questions.
Victor | Shopify Social Care