Yep, I've done it. Currently running a custom NAS build with 2 x 2GB DDR2@800MHz (different manufacturers) + 2 x 1GB DDR2 (different manufacturers as well, one is 667MHz, the other is 800MHz), all of them run on 667MHz.
Just make sure to do a memtest when you plug them in. If it passes, you're good to go ๐.
Mixing RAM is OK. I've been running consumer PCs with mixed memory for years.
Better not put different sticks on the same channel, though. Depending on your MB you may need to set frequency manually to 1333, but most likely it will work without that.
Should be fine. All the memory will just need to run at the speed of the slower kit. Make sure you put the sticks in the correct channels for dual channel support. But yeah, maybe only do so when you actually need the memory.
I've done it for old scrappy machines in the past, but IMO it's all down to how good the BIOS is...
Most should downrate to the slowest RAM speed... you might need to manually set the speed, but... it should work.
My biggest concern was the RAID I didnt even think of the bios. I might test short term and see how it goes. Thanks!