I would like some advice on what to add to my Harlequin army as a ally

[Xaric]

So I currently have two choice's eldar craft world codex or dark eldar codex now I have glossed over the two books online but what my army is currently lacking is ranged firepower as the max my models can hit at is 12" to 24" depending on the models I loose on most cases 30% of my model's die before they reach the enemy but still a 50/50 rate of wins and loses but I can see the holes in my army the other problem is I must make the ally models have a sort of "fit in count as aspect" with the harlequin fluff so I have two choice's to react with my 3x death jesters or 3x shadowseers.

[Mr.Gold]

go eldar, guardian jetbikes, vypers, and fire prisms/falcons/night spinners, led by a seer council (or just a farseer) on jetbikes, possibly with shining spears/warp spiders/swooping hawks as back up melee/additional firepower.

[Xaric]

Reason I cant take those models
Jetbikes because I have skyweavers
Vypers/fire prisms/falcons/night spinners because I have voidweavers

I can take the seer council because by the fluff the shadowseers had leaders that followed a sort of coven so I would take them and give them a sort of harlequin look.

I was thinking of taking eldar rangers and replacing there heads with harlequin heads or Eldar Guardian's or Dire Avengers replacing heads and other parts of them to give them a sort of harlequin look.

I am also looking at the Eldar Support Weapon but modifying it like how the death jester gun looks with the blade on the end

[Mr.Gold]

you could still take the fast things, just mix&match bitz with Harlies e.g. harlie rider on guardian bike... re-model vyper to more similar to voidweaver... etc.

[son_of_volmer]

Harlequins are some expensive units, pairing them with Eldar is going to get costly, try Dark Eldar. You can do more with Dark Eldar without a ton of upgrading.

If you are taking a formation from the Harlequin codex, you could make Dark Eldar you primary. Grab two haemonculus HQ's, give them webway portals and give each of them a maximum sized Kabalite Warriors warrior squad loaded with [2] blasters. Add in a Venom as 'dedicated' transport to run off and grab objectives. The No Scatter Deepstrike might not help your Harlies advance, but the units you deploy on the table could draw your opponent's attention allowing the reserves to strike where they don't expect.

Mandrakes could help you put some threatening units in you opponent's grill. Scourges can give you some range. Talos' are also sweet.

[Xaric]

damn that would cost my awesome warlord traits but maybe your right

[Houghten]

It doesn't have to cost your warlord trait; you can do everything Volmerson just said with the same Detachments and Formations but make one of the Harlequin formations your Primary Detachment instead of the Dark Eldar.

[Xaric]

ah okay also I was just looking at my brothers codex he plays grey knights there fluff seems very similar and I wonder if harlequins have ever met and teamed up with grey knights when dealing with slaanesh daemon horde?

[Ravingbantha]

In the recent Exterminatus campaign, the Blood Angels and Necrons teamed up. The only reason was because they both saw the Tyranids as a bigger threat. Why is the assassin there? Because he's talking advantage of a situation.