How I can do XFCE as default DE?

[ricklinux]

There is already another post regarding this question here: Use Lightdm as Display Manager

How to set kde plasma as Default

For plasma, I suggest SDDM simply because it is default and it is easier to match your login screen and your lock screen theme-wise. That being said, either will work. If you switch to SDDM, it will remember your last selection so once you choose plasma it should boot to plasma from then on.