dont mix bearings from different sets..

bearings are ground down in sets so their fault (how much out of spec to the larger or smaller side) is all the same, thereby giving you much more consistency for your buck.

uneven bearings not so good for your headset.

when you replace bearings, do it all in one shot, they are cheap.

this is something that was stressed (no pun intended) to me when i was in a similar situation.