You can pick anything really. But my opinion is that when you play dervish primary, you'll end up using most of your slots for dervish skills. --- Hello. I'm a hypocrite who likes to yell at women and punch children.
LeadPencil is right that, in most good setups, you rely more on your primary than in your secondary. However, I have found that Rangers have a variety of skills that can be very beneficial to the combat-focussed classes. Just remember - your Ranger skills are there to support your choice of Dervish, NOT replace them. --- "As the good archmage often admonishes me, I ought not to let my mind wander, as it's too small to go off by itself." -Danilo Thann
It's OK. But in my opinion the dervish is a battle monk. Most of the skills are spells (except the scythe line). Even the primary attribute is about casting (well specialized towards enchantments). Well you could take stuff like apply poison (really nice with scythe) or antidote signet (dervishes have limited number condition removal skills) etc.. I hope just you aren't thinking about a bow dervish - it can be done, yet it's pathetic compared to a primary ranger or bow assassin.