These are safe generalizations for older classical music of those time signatures, but as both classical and other music have evolved, players and listeners have moved on from assuming emphasis on certain beats to letting the music dictate the emphasis.
With that said:
Time signatures can always be divided by 2 or 3 or a combination thereof. 7/8 can be 2+2+3, 2+3+2, 3+2+2. 6/4 can be 2 + 2 + 2 or 3 + 3.
Even those are overly generalized, but rather than look for a rule, just know and understand the possibilities so when the music calls for one of them you can play them.
It is very important not to generalize any subdivison for any given time sig. If you are playing the "most common" 7/8 or 7/4 grouping of 1-2 1-2 1-2-3 and the music is putting heavy weight on the 1-2-3 1-2 1-2 grouping the sound will sound awkward and rhymically dissonant (yes yes some proggers do it on purpose, and im guilty of it too!) The best thing to do is jam on all of them, and when you find yourself in that musical situation, sit down with the music and try and count the different ways and see what fits in. Happy odd timing!