1. no, unless you want to be able to use the outdoor sensor and the advantages it gives you.
2. if I read it right, the 7400 does not support an outdoor sensor.
3. If that includes the price of the tstat, yes.

When you say 'gas', do you mean natural gas or propane?

What are your electric and gas rates? This makes a difference.

The idea here is that (lets say) if you have relatively high electrical rates and relatively low gas rates you can set your system to run on electricity at higher temperatures and to switch over to gas when the HP is working harder. With the outdoor sensor and the proper stat, you can set what temp the gas starts to be used and what temp the HP stops being used.

These temp settings are a combination of cost and comfort.

Very well put Weed. This type of configuration gives the owner full control of the system that is automatic. It doesn't matter if it is 5 degrees or 50 and how the temps swing in between if your not standing at the stat the system will function accordingly. With dual fuel the payoff will be quick merging fuel cost with system efficiency and electric cost.