Hmm. Do you plan on just editing the ROM to suit your vision of the custom Pokémon world? Because that sounds a lot like ROM hacking to me; except that you don't release it to the public. XD; That being said, the
Emulation forum, specifically the
Toolbox section in it has a lot of stuff that you can check out if you need help editing the base ROM itself. If by any chance I'm actually way off in this belief and you're actually talking about
Game Development with programs such as Game Maker, then I linked that forum there, too. Just in case.
As for the perfectly balanced issue, I think
this thread will be a good place to look at. I'm still not sure how the type balance can be solved without mixing or just resorting to the Dark/Psychic/Fighting trio as secondary types (a disadvantage of which is the Psychic immunity of Dark-types), but yeah, I hope you can find the solution from that thread. Oh and the thread is dead so don't bother replying there, else you might risk a thread revival. Haha.
That being said, these types of threads best belong to the
PokéCommunity Questions & Feedback subforum, specifically the
Quick Questions & Answers thread. Still, you seem to sincerely want to be welcomed to this community judging from your thread title. So without further ado, I'll go on ahead and say the obligatory welcome to our forum, JoaV! :p I hope you do stick with us in more ways than for your current worldbuilding endeavors. Because foruming is fun. Here at PC, especially. (b'')b
Maybe you can expand on that intro! I mean you seem like an interesting person because you like making custom games of your own. That takes a lot of imagination and thinking, and really, those kinds of people who got those in spades are definitely special. :o So yeah, what are your interests? You think you could tell us more about yourself? :D We can wait, or at least if you're not too open with stuff like that then that's okay, too.
I think I'm starting to run out of things to say, though. This post has been a long one in making, haha. So yeah, see you around! And I hope my welcome was good enough. ^^; If you need any help at all, don't hesitate to ask a staff member of the section related to your query. Although I think that if you follow the rules and read the FAQ, most of those common questions will be answered for you. :3