Firstly, there is no need for this topic. However, sometimes people in ignorance and curiosity may question Gursikhs about Bibek Rehit regarding this.
Daas' limited thoughts on the topic are the following:
Kumma Maashki never gave Mata Ji and Sahibzade food, it was his next door neighbour Bibi Lakshmi Devi. Bibi Lakshmi Devi, I am assuming was a Sikh but not taken "Khande-di-Pahul". Just because history states they were given food does not imply Rehit was broken.
Similarly, Baba Moti Ram was a Sikh, who probably taken Charan-Pahul but not Khande-di-Pahul. Furthermore these are not normal circumstances and therefore we should not look into this matter.
Regarding, Gangu, Mata Ji and Sahibzade never actually ate from them because the food offered was unfit for consumption. I heard in one lecture that the food offered was "Kachha Missaa" - uncooked/unprepared raw food. Gangu was definitely not Guru Ghar Chef. He was the Roseeaa (Chef) of the two Masands of Saheri - Masand Dhammi and another Masand (whose name I do not have with me at the moment). Other sources say he was the Chef of the Kashmiri Pandits.