Is it OK to use stock key caps on a custom or should I upgrade to a full new set?

I have the GK68XS from EPOMaker and was also wondering if any other sets would actually fit my board?

It’s absolutely okay if you like them. Don’t feel pressured by the community to sink more money into a board if you’re content. If you want different keycaps and have the cash, go for it. Your specific board looks like it will fit keycap sets that are advertised to fit 65% keyboards. If you select a set, pay attention to the caps on the bottom row especially and make sure they are represented.