One of the biggest changes for 6th edition is of course allies and the repercussions this has in list design and on the tabletop are huge. The hard part in the fledging stages of 6th edition is the latter is going to be informing the former but you cannot have the latter without some form of the the former. Thus you need to be relying on your understanding of game mechanics and past experiences which you can translate to the new edition before you can actually get a good cycle going. Aka educated trial & error and that's what's been going on. Allies makes this a right pain because there are a lot more combinations to consider so let's look at a few things learned so far.
KISS -
Combos for 40k are finally in due to cross codex abilities but don't over-complicate it. If Unit A needs to do this for B which does this for C which does this for D which then does this for A and that's the reason you have ABCD, stop it. A to B and maybe even C back to A is okay, particularly if you can make it something like 2A, 2B and 1C for some redudancy. The more complex your reasoning for taking allied options for a combo, the more likely it's a bad idea.
Be flexible -
Which leads right into this. I love the Necron flyerwing allied force (CCB + 2x Night Scythes + Doom Scythe) and althought it's great, it's not always the right or only choice. This is really the same as normal list building - don't get too attached to anything for competitive play. There are a lot more options out there now and you can get some of the cool toys you've been eyeing off from other armies but that doesn't mean it's always the right toy. Make sure it fits with your parent army list or change it. Be flexible just like with normal list building.
Points Consideration & List Design -
If you're taking two Troops and HQ and something else from an Allied contingent it often runs up to 700 points as an average figure I've found. You can easily do 1000 depending what allies you're running or as little as 300 but 700 is generally a good ballpark if you don't know exactly what you're taking. Take this into consideration when building your list as you don't want to end up with a 1500 point army with 700 points of Allies and overshoot the 2000 point marker quite handily. This leads us too...list design!
Which of course is hard to quantify. Do you do parent list first or allies first? Simultaneously? All are viable really as long as you have a clear idea of what you want to do first. If you have an idea of a parent list - make it and then see if there are any holes which can be plugged by allies. Find the allies which plug the hole. If you have an allied contingent you just must use (but don't want to use it as a main army) look at its strengths and the holes it could plug in parent armies. Find a parent army which could use such advantages and work on making a list to include the allies you want. Or you could do both at the same time which is generally the hardest but produces the best lists as both the parent list and allied contingent evolve as the understanding of what one is trying to do grows. This in essence is building the list with a concept in mind knowing the allies which are going to plug your holes and tweaking the entity as a whole to get the right mix at the end.
Ally Role -
The most obvious thing is to know what the Allies are there to do. Are they filling the hole of the parent army - diversifying the type of firepower available, adding scoring durability/saturation, adding mobility, ability to deal with certain types of units, etc.? Or are they just strengthing what the army does already? Both are fine - yes the second is fine, as long as the army is balanced as a whole. If your army is an excellent midfield shooting army which has combat defenses and can deal with infantry, vehicles and flyers and you can bring in allies which help with this, well if it works, it works. Just be careful when filling holes in the army that the army you're attempting to fix with allies is fixable, sometimes its just better to scratch the idea and start over.
Ally FoC restrictions -
You can only have an HQ, two Troops, Elite, Fast Attack and Heavy Support. For those of you who enjoy symmetry or look for redundancy only through repeating units, this can make allies frustrating. Even without these self-imposed restrictions, not every unit makes a good allied unit. A single Dreadnought all alone for example is, well it's a single Dreadnought. A single allied Dreadnought + parent Dreadnough with the other Elite slots used though, well that's letting allies add to the army. Otherwise look for units which work as a solo - something that if you were designing the army from the allied list as the parent, you'd be happy to take alone. Examples include Doom Scythes, full Sternguard squad which can combat squad, flyers, etc.
Remember as well you have two Troop choices so you can double up there and this is really helpful for ensuring quality scoring in your list overall. Make sure you know how these are being used in relation to the rest of your army though (i.e. aggressive or sit back and shoot). Finally, remember you HAVE to take an HQ and a Troop. To really make allies work you need to ensure you NEED both of those units, particularly the HQ. Characters are greatly improved in 6th edition due to durability and challenges but they aren't cheap and if they aren't really benefitting the army and being a good-beatstick beyond this, you're sinking a lot of points into something you perhaps don't really need.
Type of Allies -
Allies of Convience are the most commonly seen but Battle Brothers with new USR rules can be awesome for transfering unseen USRs into different armies. Don't Ally just for this but take advantage of this if you can. Be very careful on how rules interact, even with Allies of Convience. Although Desperate Allies aren't really going to be the hot stuff, they can be used as long as you remember they don't score/contest and can screw up your army if they get close to it. Using them as a completely different element to what the rest of your army does can work (i.e. aggressive in a shooting army) if you keep these restrictions in mind.
Conclusion -
There are a lot of ally combinations and there are a few stand-outs you should be familiar with (i.e. IG platoons, Necron Flyerwings, Ork bodies, etc.) but keeping all the above in mind will hopefully net a better end list result. Remember, make sure the parent list needs the Allies and that every point spent is actually being useful and bringing something to the table.
