I believe that the heads on standard allen-head cap screws may be taller than space allows for in many of the parts that stack together or for which a recess is provided (e.g. corner pieces that support the conduit rails).
I was able to easily locally source the hardware here in the US even during the pandemic. If you absolutely need to work with the hardware you have on-hand, I suggest you check clearances as you go and be prepared to improvise (e.g. grind down a head where needed) if you find things aren’t fitting.