Copying an ability simply adds a second one to the stack with all the same text as the first.
I was wrong in my first reading. With Isareth, the ability is placed on the stack before the cost is paid. If you were to copy the ability, you would need to pay X for each copy, but could pay different values
Your second example is correct, you would place a +1/+1 counter on all your creatures, twice.(Ie: 2 +1/+1 counters per creature)