When i got my car the front tires where needing replaced so we replaced the two front tires BUT the other two rear ones where sort of new but maybe had 70-80% tread left. Is this bad that we didn't get them ALL replaced at the same time?

 

As long as you bought the same size tire (which I'm sure you did) everything will be fine

 

Same size as the outer diameter?

 

If you hd 80% left on the tires on the Axle you are fine, if they were low no tread I would say DO ALL 4.
Need to keep diamater/rev per mile close.