multi codex army? legal?

Not legal, except in Apocalypse and extremely friendly games with your opponents consent.

yea you could do that if i could run Chaos Daemon Chaos Space marine And IG in one list
Slaanesh Daemon HQ with the ability to lash 4 times in a turn, 9 basalisks and noise marines

as Klokk has already pointed out, very illegal. Infact WAAAAAY back in the day there use to actual be rules for which armies could ally and which couldn't. Like if you and your friend were going against someone else he played Orks and you played Space Marines the rules dictated the two of you could not play on the same side.

oh i know its illegal i was being a smart ass with the chaos cheese move i could pull.

The Daemonhunters and Witch Hunters codexs still have allies rules, you if you're playing an Imperial army you can take GKs/Sisters/Inquisitors. Those 'dexes are so out of date, and the models are generally hard to find or expensive, that im not sure why you'd want to except to fill a VERY specific role.

The one exception is the Vindicare assassin. At 110 pts, this guy RULES!

The only way I think this could be considered is if you play a multiple detachment game. But stipulate that both the detachments have to be codex legal with a combined total equal to that of the opponent. As in minimum requirements(1 hq 2 troops minimum) for each detachment. I think the result of that would be more balanced. But once again that would be friendly games only. However it would be far more agreeable to the opponent.

Your opponent should be able to use multiple detachments as well to make it fair, otherwise you would be able to choose 4 HQ and 6 Heavy Support and your opponent only half of that, etc...

Each codex is supposed to be a balanced mix of different units and abilities with appropriate points. As soon as you mix codices you throw out the whole balance and leave the door open to powergamers.

That said if you use detachments with a HQ and 2 troops from each codex there isn't too much room for nasty surprises in a small game.

With the exceptions of apoc, WH and DH codices (and guard or any SM chapter), not normally, no.

If your normal opponents ok it, then it is fine for that group.

A tournament? See my first answer (and some don't even allow DH/WH to use their allies rules either).

You are normally supposed to use the units from the ONE codex for your army.
They have strengths and weaknesses for a reason. If you could use the strengths of any army to plug the weaknesses in yours, then what would be the point of playing?

not legal is the short arnswer but the longer one is mostly.

In friendly games and Apocalypse it would be almsot fine as well in doubles games but allying 2 togheter for tornements will get you kicked out faster than you can say "Hitler is my god".
I persoanly would allow it in a friendly game...fluffy tho it would have to be(i think i turned into yoda then!!), for example Imperial guard and Crimson fists would be fine but Tau and Tyranids not so much

so basicly, reading from your example, you want to take the "best units/characters from 2 or 3 books and make an army useing them? power game much, becourse thats how it looks to me (and no im not calling you a powergamer). the only way i would allow it is in freindly games, where i can do the same thing for my army, but probaly woundnt make for a fun/enjoyable game to be honest

I use to love allying with a friend when it fit with fluff, like your IG and Crimson Fist example jaws900, but OH did it irritate me to no end (I suffered silently ofcourse) when I would face the Chaos Space Marines and Ultramarines army or the Necrons and Eldar army, but honestly there was a thread about making up fluff to fit why each army would ally together.

Someone should revive that thread