It is always a good idea to have a team that has various different types, strengths and weaknesses. This way you will be able to anticipate for any Pokemon that comes your way and you will be able to choose the Pokemon that has the tactical advantage in your team.
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I use Ho-oh arceus darkrai jirachi groudon and deoxys
Sadly no, Nintendo tried its best to stop all the use of AR on Pokemon Platinum. But hackers found out how to crack the game. But their are many codes out there for Pokemon Platinum that are similar to Pokemon Pearl. You just have to look on the AR website.
There are various good places to train flying types in Pokemon Platinum. A good thing to do would be to use the vs seeker.
Bidoof
You can use the GTS in Pokemon Platinum after you get the first gym badge from the Oreburgh City gym.