I did not know this, but just checked and you are right. Each companion gets a free specialization. I can play all 3 rogue specs since I have varric, sera and cole already! Thanks for letting me know. Now I dont need to pick one right away. Just going to experiment with those 3. Making an all rogue party.
Yes, this is a good thing to try. If you have enough gold, I recommend buying three
The Tactician's Renewal amulets, one each for Sera, Varric and Cole. When you equip the amulet, it disappears, and all of the points previously assigned to abilities are refunded. This gives you a clean slate to work with and build from scratch. You can put all the existing points in to the specializations: Artificer for Varric, Tempest for Sera, Assassin for Cole, and be able to try those active abilities out right away, rather than wait until you level up another 8 levels. If you want to put things back they way there were, just buy another amulet. You buy them at the merchant that's inside the Skyhold Undercroft, where the forge is. It's the weird metal flag/banner thing on the right.
BTW, dragoninquisition.com is a very good place to find builds, but tbh, I think the builds posted in this forum are better. Particularly actionhero112 builds. Because here you get a better explanation for why it is a good build. And sometimes, some debate as well.
All of the rogues can be awesome. Like we said in a previous thread, DW Assassin is the easiest to get to awesome, but all of them can be, whether Archer or DW. Artificer mines/traps are just a different kind of awesome from Assassin and Tempest. If you like huge damage numbers, Assassin is what you want. If you like lots of pretty multicolor explosions which, added up, make a big damage number, go Artificer. If you like to do all kinds of different abilities in a short amount of time, or control time itself, go Tempest. But ultimately, it doesn't matter, it is pretty close to impossible to make a mistake with a rogue build.