Yes. Barracudas live in tropical and subtropicals areas.
Not sure what you are asking here, broke as in not working? or broke as in pieces of glass are in the tank? If the heater is not working, you need to purchase a new heater. If the glass tube is cracked or broken and pieces of glass are in your tank, then you need to unplug the heater and remove the glass from the tank.
yes
The part that generates heat... depending if it is a gas / electric heater
You can but it will need to be a plastic or acrylic heater, inline heater, or have a heater guard over it, otherwise it may melt the acrylic. For smaller tanks small mini heaters are available that would work just fine. Check out www.bigalsonline.com they will have what you need.
does not need to be removed
It would help as most of the rust accumulates at the bottom of the tank.
They do not need a heated tank as long as your temperature will naturally stay over 70 degrees which it most likely will. If it does not I would invest in a heater.
yes but it would need a heater as they are both tropical fish and you would need to change the water alot.
I would not recommend that you try. A block heater is preferred.
It would help as most of the rust accumulates at the bottom of the tank.
Condensation Or the tank rusted out
Personally I would not, as the cold water could thermo-shock the heater. And if you have a glass-lined tank, it will crack it and shorten the life of the tank. I would follow the manufacturer's instructions.