Is there any good reason why I should not slay Decanter Goblin's Clan, or the other Creature that has the bracers (cannot remember it's name)?

Also, When I multiclassed my ranger to priest of lathander (why can't I choose mieliki?) he recieved 20% xp penalty. Why is that? Can I cancel it?

Can't think of any reasons - they're goblins ;)

If neither ranger or cleric is the favoured class of your PC's race, then you recieve 20% xp penalty if they are more than 1 lvl apart. You can't cancel it (until their lvls match)

And there's probably some alignment restrictions on cleric of Miekki that conflicts with the one you have chosen for your ranger.

There's no reason to not eventually slay them - first do all of their fed-ex type quests though to grab a bunch of experience, then kill them.

Also I don't think Mielikki is available as a diety choice in the game, don't think it's an alignment thing.