no
You cannot get Zekrom in Pokemon Platinum because Zekrom is a gen V pokemon and pokemon platinum is a gen IV game. Since Zekrom did not exist inside any pokemon game at the time of platinums release It's impossible for him to be inside the game at all.
You can't catch entei in Pokemon black he is on a different version
you cant catch entei on Pokemon pearl. entei is a johto Pokemon and Pokemon pearl is a sinnoh game. the only way to getentei is to trade it over from another game games compatible are Pokemon diamond, Pokemon platinum, Pokemon soulsilver and Pokemon heartgold. the last two i am sure you can get entei. (i have seen it in the wild)
You need to have selected Bulbasaur as your starter to get Entei wild in your game.
yes
In Pokemon Black you will encounter the white Pokemon Reshiram, and in Pokemon White You will catch encounter the black Pokemon Zekrom. Unfortunately, you cannot catch both in one of those games.However, you can obtain both Zekrom and Reshiram in one game by trading one over to the other game. That is the only way how you can get both in a single game.
In the video games, (in my opinion) Zekrom is because he is the main legendary of a game. though in the end if you are fighting with on of them it depends on your style.
Entei cannot be caught in Red - it doesn't even exist in the game.
You must have chosen bulbusaur at the beginning of the game and have the pokedex eventually entei will show up when your in grass. If you didn't choose bulbusaur find someone who did get entei, if you have a Pokemon colosseum you get a shadow entei from the game.
You have to migrate it from Pokemon FireRed, LeafGreen, or any other Pokemon game that you happen to have an Entei in. Entei can't be caught in Pokemon Diamond or Pearl. Sorry.
Zekrom is in both games, however, you may catch it in white version.
Zekrom is part of a mandatory plot event near the end of the game. When you encounter it you will be required to catch it.