They have similar uses in conversation but aren't interchangable. However, you have used the second sentence incorrectly. It should be written as: "He is too loyal to speak bad words behind your back". "Behind you" is not a full phrase in English and would normally be "Behind your back". If you wanted to include "too much" in your second sentence, you could write it as: "He has too much loyalty to speak bad words behind your back". In a way, you are adding additional emphasis by adding "too" and "far". You could even write "far too much" if you wanted to and that would also be correct and would come across with more emphasis.

You CANNOT use: too much + adjective. The correct form is: much too + adjective. He is much too loyal to talk behind your back.