Hey there bizzak so i will jump right to it
Tau - These guys are mainly used as a shooting force with very limited close combat in the form of kroot. Anyways if you are considering using Tau then battle-suits and marker-lights are your best bet i frequently fight against Tau and the jump shoot jump tactic used by my friends battle-suits with massed plasma rifles and missile pods is a real annoyance (basically using their jet-packs to jump out of cover shoot and then jump back into cover again in the assault phase). Marker-lights are also the worst enemy of fast moving armies such as Eldar and
DE able to completely strip jink saves and also boost ballistic skill. The standard infantry of the Tau fire warriors are good as well with a long range and a good strength rifle. Tau would be a great ally to demons supporting your
CC with a lot of good shooting. However as said before if you are using these then battle-suits are the way to go.
Orks - Okay so Orks are good in
CC that's just a given. However although they have a poor ballistic skill of just 2 don't let this discourage you some of the best Ork units are shooting units such as lootas with the potential to throw out 30 S7 shots in a single turn with a unit of 10. Furthermore orks can be funny with the potential to kill your own units with things like the Shokk Attack Gun. Anyway if you are going to use Orks as an ally I would use them for their vehicles and shooting units as you don't really need any more
CC lol your demons
IG - Okay another army i face frequently there are various ways to use Guard the first is the classic Infantry spam with massed infantry and heavy weapons e.g Auto-cannons my friend fits 70 Guardsmen into 600pts with the added use of orders infantry can be a real threat literally winning through numbers, Second is the mechanised list using either Chimera's or Valkyries or both to transport heavy hitting units fast such as Shock troops, Veterans etc with massed heavy weapons e.g meltaguns lascannons. That said Guard could also make a good ally for demons with the potential for large amounts of vehicles or infantry with a more than decent potential for shooting.
DE - Although i have not really become acquainted with Dark Eldar i do use Eldar as my main army so i cant see them being massively far apart anyway being Eldar they are built for speed if you use them you want to Jet-bikes or Venom's and to try to put your infantry into raiders although open topped they move fast and can unleash a devastating amount of fire power with the combined efforts of the combined units and the vehicle itself. Furthermore you have a good potential for
CC being more inclined to
CC than their Eldar brothers the
DE come with great
CC units that can unleash the pain on enemy units although as said before being demons you don't really need anymore
CC. So your best bet is to go for fast moving skimmers and Jet-bikes to outmaneuver your opponent and pack a big punch with shooting
Anyway that being said i would suggest either using Tau or
DE as an ally both would complement your Demons
CC abilities Tau with their specialised shooting and
DE with their speed and shooting.
Hope this helps