Is the insulation and cover in place?

I removed the plaster walls in the master bedroom and bathroom, but the rest of the main area (living room, dining room, and kitchen) still have the plaster walls with no insulation behind them.

To achieve the protections the structure is intended to achieve, each series must be insulated from the others and keep separate books, bank accounts, and records.  

at least R-30 insulation and obviously separate units if it is a multi-family. 

Lower risk than pipe insulation.

The problem is that it has two asbestos insulated furnaces in the basement & it has a slate roof.

Spray foam is fantastic at eliminating noise, but even plain old insulation batts do a good job.

The problem is that it has two asbestos insulated furnaces (snowmen) in the basement & it has a slate roof.

For air conditioning in this scenario, you could install a ductless heat pump if there is sufficient demand.Regardless of your choice for heating system, you will get your best bang for your energy efficiency buck by improving the building envelope (air sealing, additional insulation), which may necessitate the addition of a ERV/HRV to maintain air quality in a tighter building ... but it may also allow you to use a smaller boiler/furnace to condition the building.

@Brian PloszayThe attractiveness of natural gas over electricity is location dependant - there are places where it is not significantly cheaper.Regardless of the fuel source for you heat, your best bang-for-efficiency-buck comes from air sealing and insulating the building envelope.