How does Warhammer 40k allies work?

So I've been going through a few bits, I quite like the idea of a Space Marine/Imperial guard mix but

Chaos has been tickling my soul


So I thought of purchasing some Chaos Deamons, Chaos Space Marines and some Imperial Guard (to represent Chaos Cultist)

If say the Chaos Space Marines were my main army, how may allies can I can take? (I presuming just 1)

And is it true you can't take heavy options when it comes to allies?

you can take an allies detachment of 1 other army (space marine main army and guard allies detachment)
the force organisation of an allies detachment is (if i remember rightly )
Minimum:
1 HQ
1 TROOP

Maximum:
1 HQ
2 TROOP
1 ELITE
1 FAST ATTACK
1 HEAVY SUPPORT

you can also have 1 fortification as well now

Hey Charlie, been thinking of going down a similiar route: Emperors Children + daemons of Slaanesh.

Chaos guard for sale!

trying to flog a space marine army as well if you go back to your original plan  

In a 2000 point game you can have 2 main detachments and for each main detachment you can have 1ally detachment and 1 fortification.

Main detachments have to both come from the same dex and both ally detachments have to come from the same dex.

Depending how they get on togeather depends if they can cast powers on one another and stuff

Awesome, I think I will have to start saving some pennys