Do I make a mistake switching rep, because I think it fits me better (looks, age, body type, etc.) even though my voice still is more appropriate for different (lighter) rep.?
Since we are still talking opera here, at the end of the day, you can't be untrue to what you are vocally. this can be tough if you're body type doesn't exactly fit your voice, particularly in today's world of casting visually as well as aurally. If the issue is weight, that is of course, something can be addressed, but if it is height or age, this becomes trickier. It is important that you continually get feedback to see if your self-perception is correct. A trusted coach, director, or someone in a similar position whose opinion you trust cam make sure you are pointed in a realistic direction.