I think Stromlings can mean any Maelstrom creature. But interestingly there is a distinction between Stromling and Darkling, with Darklings being more primitive and quite distinct in their aesthetic (and I've produced the idea that they are a corruption of the Baron). So there could be additional distinctions to be made to what classifies a Stromling. I've deviated from the LU wiki's list of Stromlings here, because I did not include Darklings, whereas on the LU wiki some Darklings are included with the Stromlings. I think they (and LU canon) define Stromling as a general term for any Maelstrom creature.
We can freely come up with other distinctions, and just an idea for us, perhaps Stromling is used to refer to species that were not originally Maelstrom and became infected, which would include the basic Stromling and its derivatives (based on a minifigure), the Mechs (based on Nexus Force mechs), Apes, Ronin even if new instances of these species are synthesized by the Maelstrom war factory.