|Smells Like Wargaming|
Continuing with the Ally theme... one of the common rules of thumb you'll see out there is to ally Marines with Xenos and Xenos with Marines (where possible) to get the best advantages of both general army types. Marines are generally quite durable and have good scoring options in this regard whilst Xenos generally have greater efficiency of firepower, mobility or that something 'different' which people like to pretend means their army is a finesse and if you don't win with the army you're not using it right. Little jab - couldn't resist! This rule of thumb has led to a lot of frowning at Marines allying with Marines or even Xenos allying with Xenos. I'll be quite clear here - it's a good rule of thumb, but it's not a law. There are many good combinations you can generate by looking to ally similar army types together.
Let's look at Marines first. Each codex has a unique difference and use their Force Organisation Charts differently. Space Marines have cheap TH/SS, can take Dreads in both Elites & Heavy Support, Bikes as Troops, Thunderfires, scoring Sternguard, twin-linked melta/flamers, Fleet to units, combat tactics, etc. Blood Angels have Dreads in Heavy Support, assault Dreads in Elites/Troops, Assault Marines as Troops with melta, fast vehicles, Mephiston, FNP/FC bubbles, etc. Space Wolves have great all-round Troops & support firepower (Grey Hunters + Long Fangs), counter-attack everywhere, excellent aggressive HQs + units (Thunderwolves), 2nd best psychic defenses in the game, etc. Grey Knights have 24" shooting, Fortitude, cheap as dirt Troops, best shooting Dreads in game, extremely efficient shooting transports, a great array of psychic power access, good counter-assault ability, etc. Dark Angels have Ravenwing, cheap Typhoons and Deathwing. Black Templars have five-man Troop squads with special + heavy, cheap Typhoons and awesome Terminators with tank hunters & two heavies per five guys.
If you can combine any of those strengths together with another Marine army to get a Marine army+, that's fine. It again goes back to what we discussed in the last Allies article - if there's a reason and a need to take the Allies and every unit you're allying with is an effective and good choice, go for it. If you're just doing it to get the fast Vindicator or Thunderwolves or whatever into your army and not looking at the whole picture, it's probably not a good idea and you're better off changing what you want to do or looking at a different army to be your allied force.
A great example here is Grey Knights with Coteaz. By running Coteaz and Acolytes in your Troops you're losing many of the MEQ advantages in terms of scoring durability. However, a core Troops + Coteaz + three Psyfledreads only costs around 1000-1100 points depending how you run it - even at 1750 that's around 650-750 points to spend on allies or 900-1000 at 2000 points. Most allies are eyeballed off around the 700 point mark which seems pretty damn perfect. Since the Acolyte + Dread combination is already damn efficient, allying in Marines for Marine durability in scoring is a great idea even though you're using two Marine books.
Another good example is using Space Marines + Blood Angels. Very similar armies but with a couple significant FoC differences. Managing the points here is a problem but you can get a nice amount of Dreadnoughts + Sternguard with a mix of slower vehicles and faster vehicles for where they are needed. You lose some of the FNP/FC efficiency from the Blood Angels side and combat tactics from the Space Marine side but with the 1.5 different FoCs essentially, you can get a bunch of different unit arrangements which aren't really possible normally. Again though, you need to be very careful with points and how this is all laid out and if you're not better off trying something else.
So let's look at Xenos. Xenos are really in the same boat as Marines allying with themselves but each of their books is significantly different from each other in that the basis of design is worlds apart. Even Dark Eldar & Eldar, kin in everything but the word, are very different constructs. This gives them a lot of options to really diversify what they are doing. Furthermore, some Xenos armies are really shafted in terms of allies - i.e. they only get Xenos Allies of Convenience or better. This is unfortunate but must be worked around.
Here the focus is generally ensuring you have enough Troops. Whilst Marine Troops have a lot more durability through good statlines and flexibility with combat squads, Xenos Troops are generally cheaper and plentiful. I'd say a VERY good rule of thumb with Xenos allying with Xenos is to ensure you have at least six if not eight Troop choices - it overcomes their statline deficiency and forces a lot of targets on your opponent. This is obviously at 2000 points and lower numbers are better at lower point values. Beyond this there are options for more durable Troops in Xenos armies - this is often done through simple weight of bodies (a 30 Ork Boyz squad isn't that easy to remove) or with a few fiddles is durable enough (i.e. 15x Hellions w/Baron + paint tokens; Wraithguard as Troops; Meganobz as Troops, 10x Immortals, etc.) to not need the 6+ Troop concept to come into play.
Beyond this, again, ensure you're gaining some sort of benefit from the allying army - you don't want to just take it for the extra Troops. Eldar is pretty good here as an example as you can get a decently durable Troop (Wraithguard) or two larger squads (Dire Avengers, Guardians or hell Rangers) and a great utility HQ w/Doom + Runes of Warding without significantly impacting upon points. It's all about making sure you are getting some bonus from the Allies AND improving your scoring power. You're not gaining the advantages of Marine scoring durability so you need to get those advantages in a different way - don't play Xenos like Marines and you'll be fine!
Really this is just an extension of what we were discussing before - make sure you really need and are using your Allies effectively. If even one unit isn't worth it (i.e. the mandatory HQ or Troop), you really shouldn't be taking it. It's fine to ally with like-minded armies as long as you are getting these benefits - you don't have to diversify and make a multi-faceted army as long as the similar army + allies combination you create covers all the bases needed. Sometimes this is forced upon you by the Allies matrix and sometimes its a choice but as long as its done correctly, it's a fine option.