Assuming I have understood the question, and that can be the first step ..

It sounds like you are using a track with a maximum rating of 63A to supply a lesser load - say perhaps 20 or 30A ?, and you propose to supply the track from a breaker more closely matched to the load ?

Yes, you certainly can, it may be a bit of a waste to use track with heavier conductors than the absolute minimum you would need - rather like using a thicker extension lead to supply a table lamp, but there is certainly no reg against it.

Arguably makes future upgrade easier if ever the load increases, and in the meantime means it all runs cooler and so is likely to last longer.

Mike.

In general it's perfectly acceptable and quite normal to use conductors (indeed accessories) rated higher than the design current - often that's just the result of physics and the practice of having a small number of standard sizes (e.g. using 2.5mm² cable rated at 27A clipped direct on a 20A circuit or a 20A switch on a 16A circuit) , other times it's to allow for other criteria to be satisfied (e.g voltage drop or loop impedances). Sometimes it's just for installation convenience (i.e. what happened to be in the van or re-using parts of a redundant higher rated circuit).

Provided Ib ≤ In ≤  Iz then it's fine (and even that's a bit flexible in some conditions, e.g. when overload protection need not be provided).

The advantage of running cooler isn't just a longer service life, but less power wastage.

   - Andy.