tyre question

need new tyres for the 90 due to two of them being MOT failures due to slashes on the inside of the tyres.
the remaining two tyres have got roughly 5 mills of tred left, maybe less.
would it be safe to put two brand new tyres on as well as the used ones, considering the difference in tred?

cheers

flash

defo, tyres never wear down evenly anyways

suppose for peats sake, stick the news on one axle (say the front for beter steering) and olds on t'other