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.
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.
I’ve been running games since the early 80s, the last few years I’ve been running the Fate system (which has no “levels” and skill advancement is a slow, gradual curve) so that makes perfect sense to me.