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 think you'll be absolutely fine with just a CSO. There certainly are advantages to having multiple (different) ovens. But it's not something you could do without. We are very fortunate to have three ovens in our kitchen: the CSO, the SpeedOven, and a 30" gas oven in the Bluestar range. While I don't use all of them all the time, having three ovens is extremely convenient when hosting large multi-course dinners. It's a game changer in making sure that every dish is ready on time. It's not unusual for me to have a dozen different dishes/sides, and time management gets complex when preparing all of these. But if that's not something you do on a regular basis, then obviously you wouldn't benefit from making this choice, and the space and money savings of only buying a single CSO can go to good use elsewhere. There are a few things that make me use our gas oven instead of the CSO. It has a much bigger cavity. This is rarely needed as I tend to debone all my roasts whenever possible and a deboned 20+ lbs turkey is so small, it would easily fit into the CSO. But when I do need this much space then it certainly is nice to be able to make use of it. Also, it has the most amazingly super-powerful broiler element. It's as close to a commerical salamander as you can get in a residential kitchen. We use it several times a week just for that. Can't get any better toasted bread any other way. Finally, while the CSO and the SpeedOven are a bit of a pain to keep clean, the gas oven doesn't show splatter as much nor does it get damaged by splatter (this is a little bit of a worry with all the complex components in a CSO; I don't want to clog the fan, the steam ports or the drain). So, whenever I prepare particularly splattery food (e.g. roasted pork belly or even just the aformentioned turkey), I prefer the gas oven. But that's mostly a question of using the best tool for the job, and with three different ovens I make a different choice from what you would make. You might just need to spend a little more elbow grease making sure your CSO gets cleaned thorough when cooking more challenging ingredients. On the other hand, there are cases that would make me strongly prefer the CSO over my gas oven. The most obvious one is any time that I am making smaller servings. The CSO preheats within a few minutes, whereas my big gas oven takes between 20 and 45min to be fully preheated. And of course, there are countless recipes where a CSO is the objectively superior tool, as steam injection is so incredibly useful. I am still discovering new uses on a regular basis (e.g. made haggis from scratch for Burns Night a few weeks ago). And I have favorites that I don't know how I ever managed without a CSO (e.g. making a big batch for fresh gnocchi). A downside with the CSO is that it doesn't cool down quickly. This can be confusing. If you are used to cook items by putting them into the cold oven, then cooking times will be very wrong if somebody used the CSO right before you. My family regularly makes boiled eggs in the CSO. Works amazingly well, fool-proof, and you can get the eggs to exactly the hardness that you like. But if the oven is still hot, this doesn't work the same. That's gotten me yelled at once or twice before :-)

@Mr Kyle I'm also interested in your question. In my home we have a non Miele 30" convection oven and a 24" Miele CSO. We end up using the Miele CSO for most cooking because it's more energy efficient with the use of space and does most everything the CO does and more. We end up using the larger 30" oven (without steam injection) for when we want to do homemade Neapolitan pizzas on a pizza steel, and for baking two sourdough loaves at a time (crockpot method or wetted mixing bowl on top method), which is our typical use case. Essentially, it's used 10% of the time. We sometimes use the MagicChef settings when we know what's going on with time, temperature, and humidity level for each step. Miele seems to have done away with sharing that information in recent ovens that have MagicChef Plus, so we would be less inclined to get something new from them.

Pizza is the odd thing out. You don't often need super high oven temperatures. Even bread can be baked just fine in a CSO. But pizza is the one exception. It works better the higher you can crank up the heat. If you regularly make pizza from scratch, look into getting an oven steel and/or make your pizza on your outdoor gas grill. Also, a good trick for making yummy pizza at home is to switch to pan pizza. It doesn't require as much heat and always comes out perfectly. But it admittedly is a different style. If you have your mind set on the type of pizza that needs high heat, then really nothing else will do.