I know this is ignorant, but can you please explain what you mean by “fittings?” And do you make a special plastic cover piece for them?
Fittings, like pipe fittings. I mentioned them here:
I just cut a piece of plastic and tape it over the fitting… Having a CNC machine I suppose I could make something fancy someday… 
There are literally thousands of different types of fittings and adapters. My solution may not be to ones liking… which is fine! Find fittings that you like! It’s the idea that makes things clean and easy to move tubing around.
Caution! Exploring pipe fittings is likely cause of insanity… 
On the subject of bristle length: say you are machining a 2" thick piece of wood, all the way through. You set the end mill and bristles at the same height at the start of the job, and then as the mill gets deeper into the material, don’t the bristles start to bunch up and interfere with things? What prevents them being eaten by the cutter?
A good brush head has the brush further way from the spindle than the length of the brush - it’s impossible for it to be eaten. The brush flattens out as things go into the stock. This is so for my Nomad 883 Pro version.
With the brush going behind the spindle there is not a lot of room and the brush is going to be eater sometimes… depends on the job. In my Nomad 883 Pro version, there is no brush behind the spindle, it’s a separate, fixed brush attached to the gantry bottom plate.
What if you set them so they are the correct height to sweep the top of the stock when the cutter is at full depth? Does that effectively negate the point of having them on there at all?
Yup, that’s pretty much a waste of time… the mess goes all over the place and the vacuum doesn’t help much.
The brush provides containment and air flow sculpting. One wants the brush to contact the stock at the same time as the end mill. Some people even make it touch a sight bit before. The brush length of 2 inches works well for most 1.5" end mill jobs. When things get fancy, it’s easy to switch to a 1 or 3 inch long brush.
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