I've been wanting to know for a while if it is possible to get all Hoenn and Kanto starters in Pokemon SoulSilver without cheats and without Wi-Fi.
If you have PKMN LeafGrean and PKMN Ruby (or their counter parts), you can use a feature to migrate PKMN up from them. All you need to do is choose a starter, catch 6 more PKMN, then transfer the starter up with 5 of the others (because you need 6 at once to transfer). Then you restart and repeat with the other starters and games.
Because you can get 1 Kanto Starter (by beating all Gyms and Red, then talking to Oak) and 1 Hoenn starter (by getting a PKMN from Oak then going to the PKMN Fan club in Kanto, then a guy named Steven will tell you that Latias or Latios is roaming the game, then you go to Silph Co. and he is there. Talk to him, he asks which stone you want (red, blue, or green) and will give you 1 Hoenn starter with the respective color), you only need to transfer 4 times.
But based on your main question, it is impossible to get all of the Hoenn and Kanto starters in game.
You would have to play through the game to where someone (could be steven, not sure) gives you the choice of either red orb blue orb or green orb, choose one, evacuate all your beloved pokemon to another game (HG/SS, Diamond, Pearl, Platinum), restart, and repeat. That is the only way i know of to get all 3 starters (Johto Kanto and Hoenn). I personally have Typhlosion, Meganium, Feraligator, Charizard, Venusaur, Blastoise, Swampert, and Blazekin. I have action replay, so i stole Red's blastoise, restarted once, obtained meganium via GTS. Sadly, Treeko is the only one i need.
Once you beat Red, you can talk to Steven in the Silph company to grab a Hoenn starter, and Oak to get a Kanto. There are only 2 ways to get all starters. You must either migrate them from another game, like Diamond, Pearl, and other previous games. The second option would be to have 2 DS's, and 2 games. On one of them, simply start the game and pick which starter you want. Once you have the ability to trade, you put the DS's together, and can trade them to the game you want, such as creating a new heartgold game, and entering the trade room in the Center, and trading them over.
My guess is to try to do the same in the beginning of the game. Choose a starter Pokemon, then restart the game to get all Pokemon. You need two Nintendo DS games, either Soul Silver or Heart Gold. It doesn't matter, but don't save; just restart. I don't know if this works because I'm trying to do it myself.
It is possible to get the Hoenn starters. All you need to do is beat the elite four and get all of the kanto badges, then go to Silph Tower. A guy will give you one of the starters; you have to pick either a green, red or blue stone.
- the green stone will give you a Treecko
- the red stone will give you a Torchic
- the blue stone will give you a Mudkip