subclass is second class
basically you will unlock second weapon slot when you choose subclass
and you can use both class skill without need keep chance weapon
downside is sublass skill will get twice cooldown
there also some skill combo that can use with right subclass but it not worth...really really really not worth that weapon combo
so choose which sub class you want to use (pistol+aoe magic?tank with heal skill?)

evolve eidolon will only increase stat and not chance eidolon it self
press N and choose eidolon to evolve and press evolution/evolve ( i forgot) and it list what you need to evolve and what lvl you need