ATCOs are trained and paid to seperate aircraft. If they include the use of FL70 when the min stack is FL80, or 7000' against something at 6000' then so what?
As long as the ATCO understands what he/she is doing, there is nothing wrong with the practice.
Any ATCO that is unable to work out the relationship between the levels on any given day, should not IMHO, be valid.
Altimetry is not hard, and we are talking about a very basic understanding to know what is going on in the above circumstances.