I think the mechanic used should reflect the attitude of the imps. Are they pissed off and have to be held in check? Do they like being in the physical world? Are they particularly stupid? I'm not thinking about the things the imps can do as much as the problems calling them up would create. Sure, calling up 10,000 would be entertaining, but calling up 10,000 imps that want to bite your head off, or like playing pranks, or aren't ever *completely* in your control, would be even more entertaining.
Maybe the imps come with a trouble aspect that the can be compelled for bad/unexpected things to happen.
Heck, maybe the imps are simply a maneuver themselves. Calling the imps would be a maneuver that puts an aspect on the scene.

Or, even better- make a stunt "Lord of 10,000 demons"
The stunt adds a trapping to an appropriate skill, probably Discipline, that allows the player to use the skill to summon minor demons. This trapping allows imp attacks (Ah, get them off!), imp blocks (wall of imps!), and imp maneuvers (imp chaos!) using Discipline.
Alternatively, the player could get a new skill called "Imp Summoning," which he would have to buy points into.
Alternatively alternatively, the stunt would allow demon themed spells, which would fallow the spell rules.

Kind of like above, but I don't think contacts fits exactly, because we already know how many demons you have contact to, and report matters less than ability to control.

...I might take a crack at River or Kaylee later...

*blink*
I don't have time to start a full write up now, but River has Cassandra's Tears. Maybe it's obvious to someone else, but it was a little epiphany for me. Apart from that it really depends what point in the series you're stating her at. If it's after/during the movie then you would have to have the whole bit, which could be represented by slightly modified shape shifting.

Possible aspects include
"Two by two, hands of blue"
"No power in the 'verse can stop me"
"It's just an object. It doesn't mean what you think"
"My food is problematic"
and my personal favorite "The human body can be drained of blood in 8.6 seconds given adequate vacuuming systems"

Playing a River character is perfectly possible, it would just require lots and lots of compels