No - Welsh does not make nouns plural when counting them. You might be able to say deg o orennau (ten of oranges) if you wanted to emphasize it though. See here: https://en.wikipedia.org/wiki/Welsh_numerals#Use_with_nouns
isio is accepted for most sentences alongside eisiau, in my experience - if it isn't, report it as "My translation should be accepted" and it will probably be added.
You will not see isio very often in sentences that Duo shows you, but you should generally be able to use it yourself.