You are thinking based on modern social boundaries. Back in the 1700s there were various Muslim, Hindu and Sikhs chiefs. Among Muslims, various Rajput, Jatt, Pasthun and even Baloch chiefs ruling Punjab, among the Hindus there were Rajput hill chiefs, Jatts in central and eastern Punjab, and among the Sikhs various Sikh misls. All of these various groups were rivals of one another. It was a dog eat dog like situation. The Sikh misls would only unite with each other when fighting the Afghans or Mughals, otherwise when that danger was over they were fighting each other. So this was the situation on ground before the rise of Ranjit Singh who united not only Sikhs but also all the Muslim and Hindu chiefs to build a powerful unified Punjab.
As for Punjabi Muslim chiefs uniting. This did occur, not once but many times. First time this occurred was during the time of Banda Singh Bahadur. After the fall of the Sikhs they went under ground. Zakriya Khan, Adina Beg and later Ahmad Shah Abdali had all united the Muslim and Hindu chiefs of Punjab in order to exterminate the Sikhs in the various Ghalugaras. This resulted in the Sikhs living in jungles, and mountains and on some occasions even fleeing to the deserts of Rajasthan. Regrouping, these misls would forget their personal differences and come back to fight united and stronger. So this was in Punjab west of river Satluj.
On the eastern side of Satluj in Malwa the main rivalry was between the Muslim Bhatti Rajputs and the Sidhu Brar Phulkian chiefs who had just embraced Sikhism. The Bhatti chiefs who were the rulers of the area considered these Jats as upstarts. Eventually the Phulkian chiefs under Raja Ala Singh beat the Bhattis and became rulers of Malwa and established the Patiala, Jind, Nabha states. But unlike the Sikhs of central Punjab, who were enemies of the Mughals and Afghans, the Phulkia chiefs under Ala Singh became an ally of the Mughals and later the Afghans. It is said that the only ruler in Malwa Abdali trusted was Ala Singh and under the advise of his Afghan generals gave the governorship of Sirhind to Ala Singh.
Same situation occurred in Lahore. Muslim leaders of Lahore asked abdali to give the governorship of Lahore to Gujjar Singh Bangi who was an able administrator and secular person who had build mosques for Muslims in his territory. Impressed by hearing this, Abdali offered Lahore to Gujjar Singh who appreciated the offer but politely decline to rule under Abdali's name as this would have made him into an enemy of his rival Sikh chiefs. Gujjar Singh was a good military chief, who extended the territory of the Bhangti Misl to Potohar beating the Gakhars from whom he took Rohtas.
BTW, Chathas assisted the Sikhs. They even served as generals during Ranjit Singh era. Briefly they were rivals of the Sukarchakia misl because both contested for the Zamzama gun built by Abdali but this gun had become a status symbol for the chiefs of Punjab.
