vIghro' luleghlu'
someone sees the cats

vIghro'mey luleghlu'
someone sees the cats

Both of the above are equally correct, since the plural is shown by
the {lu-}, and of course plural suffixes are optional, where the
context is clear.

But suppose, I want to say "someone sees all cats"; what do I write ?

Is {Hoch vIghro' luleghlu'} sufficient/correct, because the plurality
is shown by the {lu-}, or is writing here {Hoch vIghro'mey luleghlu'}
actually mandatory, because {Hoch vIghro' luleghlu'} could be
understood as "someone sees all each cat" ?

The rule is that to get the all X's meaning from Hoch, you have to use an explicit plural suffix, not that the noun phrase has to be unambiguously plural. The only correct answer is Hoch vIghro'mey luleghlu' all cats are seen.

-- 
SuStel
http://trimboli.name