QUOTE(The Ultimate Rule)
When making a new topic for help, the topic title should be about your problem and not about "I need help" or "I don't get this". Your topic WILL be deleted if it doesn't have a discriptive title!
I really like quoting this msg 50,000 times
Anyways... your questions... scarabs cannot be independent. If you want them to attack, they have to come out from a reaver. I think what you saw is just a series of scarabs moving. To do that, place a reaver beside enemy units. Set its hangar to 10, and it will start attacking that unit. Then place a location over the reaver and move all the scarabs the reaver pump out to a new location. Now run the AIscript junkyard dog at the new location and the scarabs will roam around randomly.
I don't really know what you mean by weapon upgrades, but you can reset its maximum upgrade, each upgrades increased dmg etc. (if this is what you're asking for). However, you cannot alter it during one game.
Doodads are a layer in the editor, like locations,terrain and units and fog of war etc. Doodads are an extension from the terrain and makes the terrain stick out or sink in. They are also effective in casting shadows and blocking ways or cause asethtic effects so that's why people use them.