You might want to know that there are virtually no ‘levels’ in STA, since Player Character advancement is very slow on a vertical level. Most of the time, when players increase an ability on their character’s sheet, they decrease another, keeping a balance. So, abilities are ‘shifted’, horizontally. In some instances, they can increase/add an abilitiy without decreasing another, raising something vertically. This is done by the ‘milestones’ system.
You can find some discussions about this within this very forum, one semi-random link would go to this thread, discussion milestones and the ‘reputation’ system (including promotions and – hopefully not! 
– demotions of characters) and to this thread, addressing the point of the milestone-system sometimes favouring active players a bit over quiet ones.
Thus, episodes that are ‘too hard’ or ‘too easy’ for characters (like e.g. D&D’s Temple of Elemental Evil would probably not so much suited for 1st-level-characters) should not be a thing in STA. I think that’s great, by the way. 
One does not need to think about any specific order.
But there is something, called the Living Campaign. I haven’t played it myself, yet, but I think with these missions it’s a little bit different. These, I’d play in order like ordered on the website linked above.
This is to say, you definetly can throw other missions in. The user @SSiron did this with his plans of a campaign they graciously shared with us in the forums. So it seems to work.