Will different brands work together? If on the same band and channel will my Airtronics RX work with a new 9cap?...thanks..

Most manufacturers(including Futaba)does not recommend the use of other manufacturers merchandise with their products.

Airtronics is on a different shift than Futaba.
Airtronics and JR are positive shift while Futaba and Hi-Tec use negative shift.

So in short,no, your receiver would not work with the Futaba transmitter.

While a number of non-Futaba receivers will work with Futaba transmitters, we'd by no means have a list. We do know that Airtronics FM equipment is not compatible with Futaba equipment, so your Airtronics receiver will not work with your Futaba 9C transmitter.