First I'll start off by stating, this games easy to learn. learning curve is so wide and in open book view that even a small child can get it.
Now for GD(golden deer) path for recruitment, all but House leaders and retainers can be recruited. Duh! Flayn gets accepted after saving her, her brother(official stance)<--View spoiler for the real version) can be recruited after the war starts, he is automatically accepted in after declaring war on the empire(post war)(before learning about Rhea) see spoiler 2 to know what happened to her)
Gilbert cannot be recruited in Golden deer.
Alois can be recruited Just before the war starts.(see spoiler 3)
Catherine & Shamir - lvl 20 to recruit them(not sure if its different for anyone else)
to recruit Felix without butt kissing its Flat A rank sword and around 25-31 spd.(not sure what brand of crack the ppl were smoking when they made the recruitment page)
Dorothea is the same but with authority and charm
Golden deer Retainer(Nader) cannot be recruited.(may change via april 2020 DLC)
Judith Cannot be recruited.(may change via april 2020 DLC)
wartime Only ashe leaves your party(must beat him up) keep in mind you can spare him and re-recruit him mid fight) if you want to keep him you may not wanna click fast.
if you manage to recruit all students and staff(the ones available), you'll gain access to ALL sacred beasts, meaning more special weapons to be had. when fighting sacred beasts always bring Flayn with you, have her attack the beast at least once(all of them) including the entities that are part of the side story, you may find some decent info and some hidden secrets. read spoiler 4 if you wanna know sooner than later.(perhaps more story will be given regarding this via DLC)
Golden deer path --> golden saints(Flame emblem army) not sure if its the same if you choose to just look for rhea, but its not without bloodshed(spoiler 5)
final boss is lvl 60 has 100+ HP best to level to 54-60 to beat the final boss. if all you do is grind anyways than this shouldn't be an issue.
Spoiler 1
Spoiler 2
Spoiler 3
Spioler 4
Spoiler 5