Any disadvantage to defeating Giratina in Distortion World? (Platinum)

It depends on the host game. If Platinum is hosting, it will be 60%; if Diamond is hosting, it will be 70%.

On a Wi-Fi battle, if a Plat cart hosts the game, then Hypnosis' accuracy will be 60% for both players, EVEN if the other guy is using D/P. If D/P player is hosting, Hypnosis will be 70%, even if the other guy is using Plat. (source)

Charmander, Bulbasaur, and Squirtle cannot be caught in Pokémon Platinum, likewise for all of their evolution chain. They can only be obtained by trading from a different game. You can either get them through FireRed/LeafGreen via Pal Park (a place to trade Pokémon from the 3rd generation), or you can obtain them in HeartGold/SoulSilver and trade them over.

Pikachu can be found at Trophy Garden, which is the grass behind the Pokémon Mansion south of Hearthome. They should be available at all times, if my memory serves correctly. I believe they're no higher than 20, which is plenty sufficient to get all the EVs you need.

A note, however, is that you don't need to worry a lot about levels on non-legendary Pokémon that you catch. Any breed-capable Pokémon can be hatched from an egg, which results in a new one starting at level 1. This is also advantageous as you can use breeding to get a better moveset than naturally possible. So even if circumstances result that you can only trade a fully levelled one of any of these 4 Pokémon, you can always breed it to get a low level one ripe for your own training needs.