New Englander, Summers Heat Add-On install?

[mtbrbike]

Hello,

I have a used add-on furnace and was wondering the following:

1. If my plenum has a rubber gasket (see pic) will that be affected?
2. What time should these stoves run at? My temp gauge is on the pipe but it is a triple wall chimney pipe.
3. Do you really need the 90 degree pipe in the plenum?

Thanks