Yep, they can talk
That's a pretty involved mod though, so you'd have a lot of reading.
I use a 2S 800mah in mine and have for a long time. The only disadvantage is that you have to charge it more regularly because you really shouldn't go below 7.2v.
More useful than a reverse polarity diode would be to simply not plug anything in backwards
Its more of an issue if you do choose a 2S because your balance plug is a perfect physical fit for the tx power connector... but it will fry your radio. My 2S battery has two identical plugs coming off of it... the balance plug, and one I made for the +/- to plug into the 9x. I've never made the mistake of mixing them up... afterall, one has two wires going to the plug and the other has three and I bothered to learn what the radio requires before randomly plugging things into it.
I think pretty much all the reverse polarity events that happen come from people simply plugging in a connector that fits... which really isn't ever a good idea. There are no "standards" in this hobby. None. There may be some things that are relatively consistent, but you should ALWAYS check that the battery you receive is wired correctly for the thing you're plugging it into.
In the case of the 9x's battery-bay power plug, look at the original battery holder that it comes with and you'll see which pin is positive and which is negative. Make your battery match that and you're good to go.