You can enable it, you should in fact :- ). Only situation where it should be disabled is when it affects stability, not all systems are compatible with all market available speeds.
The memory itself cannot be damaged in this way, the associated XMP profile was tested by manufacturer to work. (But not for all systems).
You can mix different brands as long as they will be run at same speed but it's usually ill-advised as it can leads to issues. It's risk, it can work flawlessly, not at all, or anything in-between. It depends on lot of factors.
In that case you might need to set everything manually because the XMP Profile can be different and be stable for one kit, but not for others (It doesn't just set the frequency and main timings, but also lots of voltages and subtimings, it can set up to 40 parameters).