CS100,
Yes it easy to do, I've done it in the past for turboprops. But to get the E190 or the F100 below 20T you would be left with 1-2 rows of seats (4-8 passengers and could only carry enough fuel for a circuit (if that). Even the operating weight of the E170 is around 22T, so it would also be a struggle to get it below 20T.