Should work. It is important that the speed matches the RAMs, but the RAMs can also run slower. So they might run a little slower, but that's all.

Actually, there are no major problems regarding mixed operation. However, one should not overdo it, with four slots not three of the same specification and one that is completely out of the ordinary. Secondly, do not build in such drastic speed differences. Thirdly, overclocking should really only be done within a very stable framework.

Mixing working memory is not a good idea. This leads to instabilities.