1. If you are asking if mini velos are designed for mini people, the answer is no. They are designed for a similar max load as their equivalently styled large wheeled brethren. The small wheels themselves are extremely strong, particularly if made with BMX components - and still very strong otherwise, owing to the simple geometric strength in a smaller wheel. Basically, there is no worry that you are "riding on kiddie wheels" that will fall apart.

(It is also a good thing that smaller wheels are stronger, because one downside of them is that smaller wheels also slam into dips and potholes a bit harder than bigger ones do, too)

2. Yes. The mini velo would have to have the appropriate dropout design for a coaster brake, which is the same of any bicycle. A stay that will hold the coaster brake arm or to which a coaster brake arm with a band could be attached to, and not an ultra thin material on the stay. Otherwise, there is no difference. Small wheels allow you to get nice low gear inches with internal gear hubs without exceeding manufacturers ratio specs, if desired. an IGH is also nice on a mini velo, because it keeps a low hanging derailleur from becoming a liability.

3. Again, dependent upon the design just as with a full sized bicycle. Mini velos can easily be equipped with racks and fenders given the appropriate design. As an example, my Cannondale Hooligan was equipped with a rear rack without issue, although the manufacturer had not added full eyelets, there were a small selection that were put to use.

4. If we are talking about 20" wheels, there are 2 sizes: 406, which is common for most BMX and folding bicycles. There is also 451, a much less common size that is used on some recumbents, racing BMX bicycles, and the odd folder.

Tire availability in 406 20" is quite good. Everything from a cheap replacement from a local big box store, to very high end tires from reputable manufacturers such as Schwalbe and Continental are available. You can purchase everything from a thin road tire to a fat knobby tire, winter tires, touring tires, folding tires - essentially, most types of tires are available in this size, and you can at least obtain a basic replacement locally without issue (unless you live in the middle of nowhere)

451 tires are not as common, nor is the selection anywhere as vast.

I've recently been drafting up different designs of mini velos for a real world production unit next year, it's actually easier in my opinion to design a nicely working 20" mini velo than it is to design a lot of other types. 29'ers for instance..

There's a lot of freedom in the mini velo design.