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I have around 1500pts of Iron Warriors and I am looking at some allies for them.

I don't want to take daemons as allies as the IW don't really have much to do with them.

I am looking at allying Night Lords using the Crimson Slaughter supplement (the CS all cause fear which is very Night Lords-ish and this also gives me excuse to use fast units like bikes and raptors).

I might take allied possessed as my Troop choice, but more likely a chaos marine squad or noise marines (Night Lords who have used warp energies to harness the tortured screams of their enemies as a weapon) with the bikes and a sorceror with a bike and the balestar relic (Konrad Curze suffered from visions during his life, so it is possible that his sons might have the same 'affliction').

I might also take them as the main army sometimes so I can take a Lord with all the nasty relics the CS codex offers.

Is it fluffy for members of 2 legions to ally in this way?

As a matter of convenience I can see the IW seeing the NL as tactically valuable (fast strike to off set their siege mentality) but the IW strike me as too cynical to trust the NL fully. Luckily the NL are quite paranoid too so as long as they have similar goals yeah. Also the IW are probably the least Chaotic of the Legions after the NL so they have that in common I guess. So joining up for a specific battle or campaign but they're not likely to be holding hands anytime soon.

I had a night lord and iron warrior army once. 3 vindicators and big squads for the IWs, chosen and terminators for night lords.
To be honest both legions are really the same, not one is more chaotic than the other. But whereas the night lords are more pirates and hunters, the iron warriors still operate as a single legion with companies and chapters.

Your list choices are very fluffy, however iron warriors also use raptors. Try mixing the units up. Like say a unit of 20 men with marines from both legions.
It would look cool.

my first thought of Iron Warriors allies would be to take guard.

maybe get yourself an artillery battery

Use the tyrants legion list for a true iron warriors list.it has the human slaves(auxiliary), big space marine squads and access to heavy artillery and tanks.
Just give your opponent a heads up.