Can one expect to find a semibreve rest in 6/8 compound time?
A semibreve rest is the symbol to be used for "whole-bar rest", regardless of the meter. A whole-bar rest is also distinguished by being written in the middle of the bar rather than being aligned with beat 1 in other staves or voices.
This exalted central bar position is otherwise only used by "bourdon" notes carrying multiple syllables in free meter, like in Monteverdi's Vespers on phrases like "ex utero ante luciferum".
'The correct rest to use in classical music theory when the full bar rest is the semi breve rest. This is always the case regardless of time signature.
This is to aid the reading of the score. If you are in a orchestra and your instrument has to rest for five minutes your life is going to be much easier if there is not several rest in each bar.
There is off course one exception when you are in 4/2 time the double whole rest is used or as it is known in the commonwealth the Breve rest.