Whether mixing DIMMs works depends on your mboard and BIOS version, hard to predict before trying it. Speed will probably drop to match the slowest DIMM in all configurations.
In principle different speed RAM in separate channels should just work, in practice some boards will only work reliably (or at all) with identical DIMM's in all occupied sockets.
For dual channel, again it might work - I have an old VIA based board that happily uses 512 + 2x256mb DIMMs in dual channel mode - but only exactly matched DIMM is guaranteed to work. Assume it won't work.
You may need to manually set RAM timings in a mixed system to make it stable, test thoroughly if you go ahead.