If you know that if the catering job won’t go well the bal simcha won’t pay you or he will spread bad information about you is. This considered hefsed m’rubah thus there is more room to be lenient in regards to amira l’akum
If it will cause a hefsed miruba, then it would be permitted to ask him to do a d’rabonon. This would not apply in every case, because not all actions are as essential and important as others. It would be advisable to have a mashgiach at the affair, to make sure that things in the kitchen are done correctly, and to answer all of your questions.
M:B 307-22, Poskim.