I haven't played SMN since SB had just come out, but Virus (edit: I of course mean Addle) was used in SMN level 70 rotation to get extra DPS out of Bahamut. Rotations and role skills are not something you should be overly worried about before level 70, though. Just read your tooltips and play the way that seems to make the most sense - and be willing to accept some advice from the more experienced players you'll encounter during your adventures!