The trick is to start of gentle, take a listen to some tracks and make a note about how they use secondary melodies, get scientific about it and make notes.
If your going to be supplying them with beats, then you need to get them to sign a contract, look for simple template contracts online, there are a few good ones.
http://hiphopproduction.com/free-music-contracts/
Welcome to the forum, what software do you like to mix with and why don't you try to pick up the sax again? there are some dope tracks with sax, "they reminisce over you" need i say more?