ASMs can definitely NOT be run externally. I believe the EuroReefs are all advertised as in-sump only, but I don't know and haven't heard one way or the other.
As far as pumps in the larger EuroReefs, I have no idea. But, I do know that if you're shopping for a skimmer for your 55 (or anything under 200 gallons, for that matter), the ASM and the EuroReef use the exact same needlewheels. The ASM uses a cheaper, and not quite as pretty, acrylic, and the EuroReef is probably a little bit better-made, but otherwise they are the same skimmer. Again, I don't know about the much larger models.