When it comes to what typ of hubs you can use it generally depends on the spacing on the chain stay. Fixies usually have a 120mm spacing as the flip-flop hubs are (usually) of that size. But any hub, be it IGH or something other, of that size will fit. Usually though freewheel gear shifter and cassette hubs (the ones that involve a derailleur) are of wider 135 mm size.
-Yes you can have gear hubs but not cassette hubs. This because of the different width of cassette hubs. A 3 speed freewheel gear hub alternative is available. Chain stay is 120mm wide.
700X25 C or ISO 25-622 will certainly fit and there will most probably be clearance even for 28-622.
At the moment we only have black or white.
On bicycles the main point of failure is usually the wheel sets. It is fairly common to have spokes failure and even to have axis breakage on cassette hubs. Axis breakage on FUBi fixie will not be likely as cassette hubs are not used.
We also use 36 spokes on the wheels as this gives stronger wheels.
Generally there are no weight limits on non-folding bicycles. On FUBi fixie as it is a folding bicycle there is a weight limit of 130kg (285 lbs).
We will also make load and special fatigue tests on much higher weights on the bicycle frame, so everybody can feel safe.
Conventional threaded, Square Taper
We have tested the FUBi system for many years. It is designed with conical backlash-free pivots, so that there will be not play.