1-2 Hq
0-3 Elites
2-6 Troops
0-3 Fast Attack
0-3 Heavy Support
Using the Standard force organization chart you are allowed 2 HQ choices.
No, in Warhammer 40k, each army must consist of units from the same faction. Mixing armies from different factions is not allowed in the standard rules of the game.
Necrons
You can't.
Following the compulsory list, 2
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.
The new apocalypse allows you to ignore the army list, and in so doing have more than 3 Elite choices per army.
A Codex: CSM army that uses only units with marks of Khorne? I would use either lots of template units or a heavy gunline.
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.
If you are serious about Warhammer 40K you can only play Orks.
3 units can be chosen from your elites....however some codex have rules that ignore or alter this rule.
No new Warhammer armies come out or ever will come out. The ones released will stay released. If a new army came out it would coincide with all the other armies' storylines and units and it will wreck the entire concept.