I have to add heatsinks to two rows of mosfets 13N50 and 20100CT on a battery charger, firstly is the hole at the top on these for a screw to bolt to a heatsink and second can they all be in contact with the same heatsink or do they need to be electrically isolated from each other?

You have guessed correctly the I know nothing about electronics and it's not my trade.

 

You have two choices:

1. Add thermal pad isolators between the transistors and the heatsink.
2. Use two isolated heatsinks, one for each transistor.