GPLed Stella license says it fine for me to profit on my own code changes as along as I make the Stella changes available (which I will).
Yes, this is (mostly) correct. To be precise, you need to release all code that allows one to completely rebuild the binary as it exists on your device. For example, one can't just release the source and keep some 'secret sauce', nor can it be obfuscated or use a proprietary build system that nobody else can get access to. Put most simply, if I (as the Stella maintainer) wanted to build the app to place on my own device in exactly the fashion that you have on yours, the released code should include all the required files to do so. I just want to go on record saying that I fully support all legal Stella-derived ports out there. And if you choose to use Stella as the base for your project, I will be checking to make sure everything is GPL compliant.
As long as you don't include the actual ROM data directly into the executable. Doing that would force the ROM source to be released under the GPL as well. This actually happened with an old Windows port of Stella; CyberStella. The author includes ROM data as header files in Stella, but wanted to keep copyright of the ROMs. That couldn't be done, and the release had to be pulled. If you include any data into a GPL app at compile-time, it automatically becomes GPL as well.
Finally, if you do use Stella, I would request that a mention be made somewhere that the project is actually based on Stella, and perhaps add a thanks to the Stella Team. This isn't required by the GPL, but would be a nice thank you, especially considering that the emulator authors (myself and Brad Mott) won't be getting any portion of the pay