How do I set a custom SMS tone for a single contact?
Go into your contacts.  Open the contact you want to personalize the message tone.  Click "edit".  Then scroll down to the bottom and click on "ADD ANOTHER FIELD".  Tap to put a check mark beside "Message tone" then tap "OK".  Then tap on "Message tone" and your options should come up e.g. Media storage, Zedge, etc.  Tap which option you want to choose the message tone for that person.  then choose the ringtone for them and tap "set ringtone".  Make sure you then "save" that contact.
That worked for me with my Samsung Galaxy S5.  Hope this works for you.