The Thermador combi oven might match Miele style a bit better than the Gaggenau. As far as I can tell, the Thermador is missing two features of the Gaggenau ED220-series: 1) not plumbed, and 2) for combi-mode, it seems to only have one level of steam available in combination with the convection mode, where the Gaggenau has 4. You and also press a button for up to 8 seconds per push, to add extra moisture in combi mode (maybe for breads?). It still does lots more than the Miele steam oven. I would have to do lots and lots of steaming to buy the Miele rather than just using my cooktop. Here is a link that might be useful: Thermador combi oven

First of, congratulations on your good deals for these three units. They will serve you well. For ventilation and wiring purpose, you will need a gap at the back of the shelf behind unit. For combi steam oven, this is where the oven sucks in the air in. It will vent warm air to the front underneath fascia. So following diagram #1 looks fine. For the sentence "The cabinet housing must not have a back panel fitted behind the installation niche", this must be from a newer manual of the combi steam oven. For example, see newer 6805 manual vs older 6805 manual. In a version that I have it uses "Make sure there is no back wall behind the building-in recess" which means that there should not be a cabinet back wall right at the cut out area. See picture for air intake in combi steam manual in installation section. In terms of installation each is 18" tall, just need to be arranged such that coffee machine is not too tall, and speed oven not too low. Perhaps somewhere where the bottom of the combi steam oven is a little bit below bench top level. A combi venting warm air at the front is not an issue for comfort. I have only combi steam oven in my kitchen and its bottom is at the bench top level which is very convenience, but that would put coffee machine a little too high up, if it is going to be on top. Here is an image from mile-vs-wolf-steam-oven thread. Found a funny image on the web of Miele ovens, noted the coffee machine location.

I had the exact same dilemma as the OP. We opted to go with the speed oven over a regular microwave which is located under my combi steam. We also have the M touch wall oven with moisture plus. With the two of us - we use the speed oven quite frequently in conjunction with the combi steam as it heats quickly and has full oven capability. This allows us to avoid heating the large oven for just one dish. We have used the speed function a handful of times and it works well but speed cooking is not something we have a need for at this point in our lives. I can also use it as a "warming" drawer if needed. It depends on your budget and if you want to spend the extra $$, but we are happy we went with the speed instead of a microwave.

I want the broil feature and only those two offer it... wolf does not. Since i will be putting the cso with a single convection under it and another single convection under a cooktop next it I don't think there is an option to mix appliance, While Miele's CSO has a broiler element, it's only about as powerful as what you'd expect for an "average" residential oven. I find it works to give dishes a little bit of heat from the top, but it isn't really very useful for char broiling. That's a problem with a lot of ovens. If you want to get a good broiler, you need to buy a higher-end gas oven (those often advertise what they call "IR" broilers), or you need to buy a Bluestar wall oven. That is one of the few electric ovens that is known for having a very good broiler element. Putting a wall oven under a cooktop doesn't generally work all that well. Not only do you have to negotiate the clearance requirements between the wall oven and the cooktop, there also is the problem that wall ovens typically sit closer to the floor. With in-wall installation, that's awesome. But when you are already in a bottom cabinet, that means the door is too close to the floor and it's very awkward to access. As for mixing and matching, I honestly wouldn't worry too much. Nobody is really going to notice after a few weeks. It's a little more noticeable when the units are directly next to each other. But even that doesn't matter much. We have a stack of Miele CSO and SpeedOven in the wall, and a Bluestar range right next to it. I don't think anybody has spotted the different brands yet. We also have a paneled SubZero fridge, but we installed a Miele handle, as it happened to be cheaper and a design that worked better for us.