You absolutely can mix and match RAM sticks, as long as their specs are the same. It’s not necessarily recommended as there might be compatibility issues, but usually ends up working just fine.

I think mixing RAM sticks is mostly fine today. Maybe you won’t get 100% performance but I don’t think it will be very noticeable. You may still run into issues with some capacity combinations depending on the mainboard/cpu. Regarding clock speeds usually all run on the clock of the slowest one.

Matching RAM latency also matters for performance.

When using different capacity RAM channels matter so take care on the order of population.

This really depends on your exact setup and might not be true for hardware raid.

Generally speaking, I would look into replacing that ageing raid with something more modern like Btrfs or ZFS where you are significantly more flexible with the drives used.

Not that I’m aware of, but I’m not as knowledgeable as someone else. I’m hoping some quick research or someone better can help. If I remember I can look later on this evening, but can’t at the moment.