Well, u can always mix and match the various ranges (15mm-28mm) to suit your needs, you'd be amazed how well that can work.
Liberation miniatures has a nice selection of varied kit for 20mm U could look at for instance. I myself have a load of 15mm Phugs at the ready to be painted up just for this cause, and the Wardroid from Hydra will be a fine enemy for my 20mm due to it's size as well (ignoring the fact for a minute I painted it up as a 1937 German one). For Robots I have the 15mm Wardroids from Peter Pig, as well as GW bits for the hoverdroids and also a mixed lot of Paranoia robots.
Don;t be bound by something called scale. That mediocre thing in 28mm gets real scary really fast in 20mm, if your imagination thinks it fits, go for it.
Johan