1) sometimes it's the same (to start a subordinate clause which gives a conclusion), but "so" can as well mean: like that, that way, and "also" is sometimes used if you speak and try to think for arguments at the same time. 2) No, there are some exceptions. E.g. if you make general statements (Öl ist teuer. Autos sind schnell.)

Beispiel : Person 1 " Wir koennen die Teile reinigen und wieder verwerten." Person 2 : Gut , so koennen wir das machen , also brauchen wir nichts Neues kaufen. English word for "also" is "thus", not very often used in common language.