do all bathroom light fixures above mirror/vanity have same cut out in wall?

I am wanting to change out my 4 light vanity light fixture with a new one that centers over the mirror. I know they all have backing plates but question is, is the hole universally the same behind the backing plate to screw it on? This is a newer home.

and also, can I just turn off light switch to bathroom light or do I need to turn off breaker switch before changing?

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