Well how the game works is by an ally system, in which each allied contingent has to be lead by a hero from that contingent, so yes you can use legolas but you need to have a galadhrim hero too, otherwise there is no-one to lead the galadhrim. I would recomend getting hold of legolas, he is a VERY useful model that is very cheap on ebay, and is important in any good army, should you choose to start a new one
However if you are strapped for cash haldir is a very good "shooty" hero, but he can pack a bit more punch in combat than legolas thanks to his armour and elf blade
The only major reason legolas is considered to be better is because he can shoot a single arrow into combat which automatically hits, this is a very useful skill that makes legolas one of the few models capable of shooting while plenty of fighting is going on, only evil models can do this, but they risk hitting their own models, unlike legolas.
As for your friend, i find that trolls are quite nice for negating the F5 of elves, but trolls can be made redundant by an elf stormcaller. As for the advice about uruks... i would ignore it, uruks cost a little more because of boosted strength, fight and courage values, but the increased fight value is useless against elves, so morannon orcs would be a better choice, since they have better armour and strength, but cost a point less
Morannon orcs also have access to mordor trolls, which have fight 7, essential for squashing annoying elf heroes, whereas uruks have isengard trolls, which are fight 6, so they can still beat up elf soldiers, but they are on equal ground with an elf hero!