Why does chwech not drop the ch at the end when it comes before ceiniog like it does when it comes before other nouns (like chwe munud)?
It should. And the ceiniog should mutate, too:
Thank you. Hopefully someone will report it next time they come across this question.
I thought money tended to use the vigesimal system?
Would that be un geiniog ar bymtheg ar hugain?
It will not accept 'thirty six pennies". Surely this would be correct also - if you were counting the number of pennies you had, or would this be said some other way?
Yes, 'thirty-six pennies' would be fine if you were counting them out. It will be added to the answers shortly.