(KGT 33-34):

Children seem to be aware of the existence of the inherently plural forms, however, for they
use them as well, though usually with the suffix {-mey} superfluously appended: {chamey} (torpedoeses),
{ngopmey} (plateses).

So, as I understand this - and someone correct me if I'm wrong - it
isn't grammatically wrong to add a plural suffix, to an inherently
plural noun; it is just that the resulting word/meaning is
strange/awkward.

It is grammatically wrong. Plateses and torpedoeses are grammatically wrong. They're awkward because it's grammatically wrong.

-- 
SuStel
http://trimboli.name