You know it's funny, the Muslims of the area actually considered the Sikhs as liberators rather than an attacking force. The local Punjab was already occupied by the Afghans when the ranjit Singh took over Lahore. During the war, Hindus and Sikhs went into hiding when the Afghans came, but Muslim villages were plundered as well (they thought the Afghans would spare them. After the Afghans lost at Amristsar, they again went through Muslim villages and plundered, antagonizing any belief that their religion made them brothers. In fact, many residents of Lahore made pleas to Ranjit Singh to conquer the city from the Afghans. So in short he was generally considered a better option to the Muslims of the Punjab than the Afghans who occupied them.

As for the empire, I'm sure there was some violence from religion but the Sikhs never forced someone to "convert or die" as the Mughals did. They also adapted with practices of the local population to keep them relatively happy.

Source: both books by Kushwant Singh "A History of the Sikh people" and "Ranjit Singh: Maharaja of the Punjab" (also with a little Wikipedia for a refresher :) )