I have some query regarding these below switches

SG350XG-2F10
SG550XG-8F8T

Please confirm stacking in those switches. As per data sheet they have described stacking using fiber ports but can we do it using copper ports?
Further, please confirm whether those copper ports will only operate on 10g speeds or we can use them for 1g links?

Can we use those rj-45 based 10G ports for 1G network devices or they have fixed speed of 10G and won’t auto negotiate with 1G device?