Fellowship is going to be helpful mostly within an academic place.
Ultrasound trained people really don't have that much to do at a community place. You may be able to set up a QC program or something similar with scans done by the EPs, but realistically, most EPs are coming in to work their shift and then go home. Plus, radiology is trying to stuff the ED U/S back in the bag...especially once we try to bill for it. It wouldn't surprise me one bit if many rads departments demand that they do the Q/C if such a program exists. HBUSM has more applicability to the community setting than one would think. Wound care is huge potential money. A well run collaboration between a vascular surgeon and HBUSM trained EP can be a gold mine. Bent patients and CO will be the exceptions however, most diving will be for wound care. (Did I mention that wound care is awful?)
Administration can be useful, but you probably want to have a degree that goes with that fellowship. There are ways to get the same or even better credentials just as quickly or faster (MBA, MHA). Administration probably has the broadest appeal, but you will only appeal to a place that is looking to get someone into administration.
As for academia, Tox and US are probably the easiest to use to get your foot in the door. Most programs have a tox guy or two lying around and it is a recognized part of EM training. Moreso if there is a Poison Control Center associated with that hospital. The ACGME is very interested in US. Pretty much every program will need someone who is ultrasound trained in order to put together a training program and ensure the residents are getting a sufficient US experience.
Peds is less useful and unlike the others, probably pigeon holes you more. EMS isn't that interesting to most programs although I'm sure many recognize the utility.