Correct me if I am wrong but;
CORRECT: Pump oren
INCORRECT?: Pump orennau
I thought it was only after small numbers -- ten or so. But also that it is not incorrect to use the plural. So 'pump oren' or 'pump orennau' would be ok (but the one with plurals sounds odd).
If you are going to use the plural noun then you need to include 'o':
Pum oren (note, pum not pump before nouns is the rule, but sometimes ignored before vowels))
Chwe afal (ditto, chwe not chwech)
Pump o orennau
Chwech o afalau
One challenge with using the 'deg/ugain/cant o..' form is that you need to know the plural form of your noun, and Welsh has many, many ways of forming plurals!