as long as they're both DDR, you SHOULD be able to mix speeds. I have one stick of 2100, one of 2700, and one of 3200 at the moment
they all run at the speed of the "slowest" RAM, however.
no harm in buying RAM too fast for your motherboard, except perhaps price. there CAN be harm from buying something like RAM on ebay
if you motherboard seems to beep in some sort of sequence, write it down and look up troubleshooting for that motherboard. most motherboards have error codes listed if you look hard enough.
ECC is an error-correcting system on some RAM... mostly for servers.
best action right now: try decoding and looking for your motherboard's error codes.
then, see about the possibility of a refund.