In principle it's not much different than mixing HDD or any storage types. System memory (RAM) or other add-on cards can have weird interactions and sometimes just don't mix well but storage just kind of is (as long as it's working).
It won't slow anything down; if the extra space is taking the place of a spinning hard drive that part will be dramatically sped up. On the other hand I wouldn't expect an improvement in performance. If you need more space for apps or games it can provide a fast place to put it. ff
Combining them as in a RAID config could boost the overall speed but drives of different capacity aren't recommended for that as you wouldn't be able to use the extra space of the larger drive.