(Advice from total julia novice follows, so may be off the mark!) Yes, you should be using composite types; they are structures.
What abstract types offer you is the ability to write general code which works efficiently (thanks to the julia compiler) on different concrete types, so they should simplify code overall. (Generic programming.) Fields in your composite types don't have to specify a particular type, ie: they can be type Any. You can start writing your code untyped, then make it type stable later where needed.
