It's disadvantageous to stick to a single Pokémon, but not all that impractical. There was once a run of Pokémon Yellow, I believe, done with just a Pidgey.
You don't need multiple high level Pokémon, it simply helps. Type coverage is the main reason - for example if your highest level Pokémon is just a Fire type and can only do Normal and Fire attacks, then you stand a hefty disadvantage against Rock types who resist all your attacks and can deal double damage. As well, Fire and Dragon types will block you completely, while Water types mostly block you and also do double damage.
Having your other Pokémon be of equal or near level as your highest helps in survivability of that other Pokémon as well. For example, a Ground type with high defense like Sandslash can help protect against the Rock types while also taking them down. This also gives you the opening to fully heal your primary Pokémon in between the assault of your opponents - it's a lot easier to keep your primary Pokémon in top shape when your supporting ones don't fall down in a single hit. At this stage, though, it probably helps to make those "assist Pokémon" simply capable of taking down foes on their own right.
It's ultimately up to the preference of the player, though. It's fairly common for people to stick solely with their starter of choice, and by sheer force of their levels take out all opposition. As long as you have good type coverage with it (for example, Charizard can use the TM to learn Earthquake to help mitigate some of the problem foes), enough of a level advantage will keep you on top.
You can generally find all the information you're asking for at Bulbapedia.
For example, here's the exact damage formula.
Speed directly dictates who goes first; it's only random in case of a tied speed stat.
Natures always increase one stat by 10% and decrease another stat by 10%, or have no effect.
Additional answers:
- Base stats are simply used in the formula for determining current stats. They don't necessarily correspond to stats at any level at all; they are just numbers that are meaningful relative to each other.
- I prefer Psypokes. For each Pokemon, you can change to see their stats, levelup moves, TMs, etc. for any generation where they existed.
- Bulbapedia has a list of all moves, sorted by generation in which they were introduced.
- Psypokes will show you moves by level, but for stats you will have to use some formulas from Bulbapedia.
Best Answer
This isn't going to work at all, as there is no way that I'm aware of to trade/transfer items in Gen 1 games, so your master ball from the Red run is stuck there. While I have heard there is some sort of item storage in Stadium 2, my understanding is that it is also tied to your trainer ID, meaning items stored in this way cannot be withdrawn by any savegame that did not deposit the item.