If you are serious about Warhammer 40K you can only play Orks.
You'll find the answer in the Warhammer Rulebook or in the army book you are playing.
AnswerYes! In Warhammer any army can beat any other army. All it takes is some good tactics and a bit of luck.
AnswerYes.
Necrons
A warband if you play warhammer
There is no "best" race or army in either Warhammer or Warhammer 40,000. They are all equally balanced so that players will not choose them based off of which is simply the strongest, but by which one best fits their playstyle as each army has a very distinct way of fighting and everybody is sure to find at least one that fits them.
No, but it always looks a lot better if your army is matching, and your opponent with appreciate it.
Confirmed by Games Workshop to be Blood Angels.
There is none! It depends on your stratagy and your tactics!
In warhammer terms TAU is a young yet sophisticated army which includes kroots and fire units. Personally I believe Tau are the best with Eldar in warhammer 40k.
You can't.
No, in Warhammer 40k, players are generally not allowed to mix factions when creating their army. Each faction has its own unique units and abilities that are meant to be used together for balance and thematic consistency.