First off, it is always safest to purchase the correct charger from the manufacturer. That way, you know it will be correct. However, you can usually use others.
The laptop will only draw as many amps as is needs, so more will not hurt. The voltage is lower, but 19.5 is only 1% less than 20, so it should be fine. 5% up or down is usually acceptable, but there is no guarantee. However, if the barrel jack is smaller, as you mentioned, it will not fit. You would need to purchase an adapter for that one to fit. Personally, I do not like barrel jack adapters, as the added length means its easier to torque and break the power connector inside the laptop. Also, its a small part and if it falls off, its easy to lose.
Another thing to note is whether the charger is center negative or center positive. Usually on there is a symbol near the plug for the barrel jack port that looks like one of these:
If you cant find the symbol on the laptop anywhere, you will need to check the laptop's manual, website, or call their support. The power supply you purchase needs the same polarity and will have the same symbol on the laptop on its label. It is possible to modify a charger to swap the polarity, but that requires cutting the wires and soldering them on in reverse. A charger with the wrong polarity will not work.
