What a coincidence haha, I've been going over Axe so I could make a game like this.
I wrote a BASIC Digimon raising game about a year ago, it wasn't graphical but it had all of the digimon from the first digivice, and more or less the same evolutionary patterns. You could feed them, clean up after them, play with them, train them, and rest them. IIRC I had some simple attack/defend battle mechanics for the fights. I was almost done except for programming the linked battles, which is where I stopped, because I then decided to rewrite it using xLib and make it graphical. I got most of the sprites into my calculator, but never really got off the ground with it, which is why I've been studying Axe recently to try it out again.
Anyway, there was a site I used by a guy who was pretty into the digivices, he had a lot of good information about the way they worked. Unfortunately he seems to be doing some renovating of his site and he hasn't put any info back on yet. In any case, give him a ring if you ever need to know how something worked. His site is http://www.digimonpet.co.nr/, and he's fairly active at its forums. Also, WithTheWill could provide any other information you need. Good luck!