lastchancename:
I’m guessing the processor will run several times faster than the compound of the step demand, so there should be a lot of leeway to interleave mixed/different step rates.
I reckon it would be a good idea for the OP to set up a realistic test to satisfy himself that that is true.
The Arduino won't just have to generate pulses. It will need to figure out when the 4 output pins for a motor need to be updated and which pins need to be high and low. And it will probably also need to collect commands that tell it to change its behaviour.
...R