RAM will default to the slowest speed present.
That being said,m all RAM is not thje same, and there is more to compatibility than speed.
www.crucial.com to see exactly what you need. Guaranteed compatible, and lifetime guarantee.
Some people just go there and buy elsewhere, but it's got to be an exact match, and there will be no compatibility guarantee. What are you going to do with memory that doesn't work if you got it elsewhere?