In general this is OK. The one thing to check is the size of the wire coming from the MCC bucket stabs to the line side of the breaker. The standard practice is to size these jumpers to match the breaker that the factory installs. Some plants specify when they buy MCCs that these jumpers are sized according to the largest motor that could conceivably be connected to that bucket. i.e. if we buy a size 3 starter bucket with the breaker sized for a 30 HP motor, we would ask for the jumpers to be sized for a 50 HP motor since that's the largest motor we may want to run from this size 3 starter in the future.
Yes, in general the stab assemblies are pre-made by machine and they don't really have that many sizes, they just use the cable size for a particular bucket range. So since the smallest bucket can have up to a 125A feeder, most MCC mfrs just use wire on the stab assemblies that is rated for connecting to that even if the FCB or starter is smaller. but you should check first.
Also, UL listing is an issue if that's important. You cannot modify a bucket in the field with regards to power devices and not void the UL listing of that bucket, and if you install a non-listed bucket into an MCC, it voids the UL of the entire lineup. That may or may not be an issue, but even if this isn't going to have a permit and be inspected, you need to check with your insurance adjuster.
