I know you can make special arrows by combining a special arrowhead with a arrow shaft. but can you also change normal arrows into special arrows? like normal arrowhead + arrow shaft = normal arrow then normal arrow + fire scrolls = fire arrow?

There is no item called normal arrow. The inventory menu is already quite a pain to use, no need to make things more complicated.

Actually there are. try combining a normal arrow head with an arrow shaft pretty sure i got normal arrow last time i did that

Well you can combine poison vials with arrowheads
Also any debuff ability vial can be combined with arrowheads. But I don't think any of these will combine with a regular arrow.

How many special arrow types are there anyway?

made this page on the wiki it has all of them i think. with stats. just a couple icons that need to be updated ^^ http://divinity.wikia.com/wiki/Divinity:_Original_Sin_Arrows