I think when discussing the ''Sikh'' Empire we need to clarify a few points;
It is basically considered a ''Panjabi'' empire with the figurehead and some of the top echelons belonging to the Sikh faith, its army had a considerable heavy contingent of Pasthtun fighters in its ranks; notably Khattaks and Yusufzai's as well as Hindokowans. Otherwise it would have been next to impossible for practitioners of the Sikh faith, on their own, to have taken over these areas.
The empire itself, was a multicultural and multi-faith based one, secular in ideology. It must be remembered that Sikhs, essentially a reformist sect of the late 17th century, where never a large population but where always a minority faith practiced by the people inhabiting the eastern Panjab region; with many of them converting from hinduism but having still maintained their Panjabi ethnic traits.
Furthermore, while the new ''sect'' had gathered the bulk of its followers from eastern Panjab east of the current border, after the signing of the Anglo-Sikh agreements, the Panjabi-Sikh empire where to push west from the sutlej/ravi border away from British colonial india into territory they lacked any base in. The bulk of its army was still made up of Panjabi's and Pashtuns; nearly all of whom where Muslims, many of its generals where European ex-mercerneries many of whom fled from their native lands after Napoleans downfall (eg. Napieri, Ventura).
Incidently, with the independence of Pakistan, and later india as well as the partition of the Panjab province whereby 4 of the regions great rivers went to Pakistan and 1 went to india, the Sikhs were concentrated for the first time into a homogenous region in india; something of a feat unknown to them prior to this. A feat that brought the vast diverse minority Sikh's from dispering regions into a more compact one. It proved to be beneficial to them, but this was to prove short lived.
Unfortunately for them, they're high rate of male immigration/refugees to foreign countries and the influx of large number of non-Panjabi's into their province in eastern Panjabi into their last bastion, is further weakening the foothold they made. The region has one of the largest inversion of the male to female populations in the world further altering the genetic make-up of the sikh panjabi's as many have had to marry outside due to the lack of available women. I think at last estimates, they represent around 60% of the population but are projected to become a minority in their own province withing the next 25-35years.
Incidently, Ahmed Shah Abdali, the Afghan king and founder of modern day Afghanistan, was born in the southern Panjab in the city of Multan. He made many forrays against the Sikhs, but had to cope with uprising and the ever present Persians making inroads from what is now Iran.
The point of all this, is while we discuss the ''Sikh'' empire, we must understand that it was a multicultural, multi-faith and secular based empire. Its rise coincided with the suppression of the Moghul empire in South Asia by the British and their subsequent tacit approval of thier rise in the first place, and the infighting and dissaray in Afghanistan proper which allowed this hitherto minority sect, to gain a short lived empire in the important and strategic Panjab region.
So while the ruler of the empire may have been sikh, the empire was a secular one and the majority of its subjects, troops and administrators where not of that faith.