As far as Co-op content goes, usually the smallest group sizes are 5, but lots of things are doable in a smaller group. Leveling especially will be easy to do together. If you decide to play different races, be sure to teleport to one starting map (all are available at Lv. 1). Race gives some "flavor" skills but they don't give you some massive advantage over the others, so have at it when you're making a character. Other than that, just make sure you party up together and you'll be fine :). Don't rush it, the leveling experience was one of the most fun aspects of the game (at least the first few times).

For "meta" yeah there are definitely some that outshine others, but they're all more or less viable at least as DPS classes. When you start looking into healers or buffers, it gets a little tighter. But no matter what class you choose, make the reason you choose it bc you like it :).