Smaller nozzel use?

You can use any size E3D V6 nozzle with the Robo, but if you go smaller than .4 things get a bit tricky. I would say below .4 is expert use territory. .25 gets even trickier if you're trying it with a filled material because it is more likely to clog.
I'd advise considering .25 only for unfilled materials. This would mean that you don't need the hardened steel version and you can stick with brass.
If you want to do filled materials, go with the .4 or the .8. Personally, I always use .6 or .8 for filled materials as it makes things easier.

 

I concur. I have .25 and a .3, I seldom use the .25. I keep the .3 nozzle on the detail printer most of the time and even then you are going to sacrifice speed for detail. The .25 is a really slow printing nozzle if you want to use it correctly.

For detail that fine I defer to SLA or DLP machines since that is their forte (and they are a lot faster at it). If you are doing that level of detail only on occasion then the FDM (the Robo for example) can do it, but it takes testing and tweaking to nail it.

 

If you are making drone parts I'd suggest the 0.8mm nozzzle (unless you are doing micro drones).
That is what we use and it makes really sturdy parts. CF and Nylon are good materials for that (just be aware that most nylons tend to be flexible so test whichever one you are going to use). I use CF and Alloy910 for most of ours.

 

You will need to adjust it, yes. You need to experiment and see what values work best for you,
For me it was almost a linear relationship (1/2 the size, 1/2 the speed).

Smaller layer heights take longer to build a model so even if the speed were left the same it would still take longer.
You need to adjust the layer height because at some point the nozzle can't do a good job at a given layer height.

My rule of thumb is to start with layer height near 50% of the nozzle size (your 0.4 nozzle is being sliced most likely with a 0.2 layer height). Again, experimentation is often needed to find the best setting.

 

Further discussion:

http://forums.reprap.org/read.php?70,100359,102031

http://reprap.org/wiki/Triffid_Hunter's_Calibration_Guide