You can keep a city free of polluting industries, but they have to be placed somewhere (ex. another city unless you get magic tokens from Omnicorp). There are measures you can take to reduce air pollution however.
Unskilled workers work in nearly every industry or city government. In the higher tiers they are needed less, but granted they are always a good population to have around as spare hands. If you have too many just export some of them to another city/omnicorp until your industries/retail/infinfrastructure catch up. No polluting industries necessary in city limits if thats your preference.
So yes it can be a lot more diverse than farms/hotels. For example you can put them to work in offices/power/waste/oil/retail/city services/leisures and so on.