Well for one the satellites wouldn't have enough fuel on board to do orbital plane changes, to at a minimum its going to be 20 launches if they want to hit 20 different orbital planes. I have yet to find out how they expect to do station keeping with that low mass.
Unfortunately, small thrusters are less efficient than large ones, overgeneralizing of course. There are a number of components that cannot be scaled down indefinitely.
Although there are other potential options (EDTs, etc, etc) that don't have as much difficulty with scaling.
Except that you'd still only need that same 1kg thruster on a 500kg satellite that you'd need on a 10kg satellite. Which is what I was talking about.
With satellites and chemical thrusters, it's pretty much "the lightest possible". Thrust to mass ratio is almost irrelevant. Almost any thruster will have the thrust required.
And I'm kind of surprised by the ion thrusters. I wouldn't assume that they had the thrust required to stationkeep in LEO - the drag in LEO being as high as it is.
First, this is extremely high for LEO, ~1200km. Drag is negligible, decay will take thousands of years.
At around ~800km, solar sails start to become practical because aerodynamic drag drops below photon pressure.
Ion thrusters and photovoltaics are COTS techs which are vastly more powerful than solar sails, which exist only at the prototype/demo stage.
Ion thrusters are practical to fight drag down to around ~200km given the right design, or are perfectly happy at 300-350 given a typical suboptimal design. GOCE managed it down to ~230km, after performing a multiyear mission at ~260km: http://www.spaceflight101.com/goce-re-entry.html
