Thanks for the comprehensive replies guys, I was starting to wonder if I'd been too lazy asking for your help!
I like Justindo's "representative unit" idea, I think this is very much in the spirit of Playmobil, and will adopt it. A 20 man century sounds very good, and I'll adopt that standard. 20 Klickies lined up in a 4x5 formation looks pretty impressive. You missed the Optio and Tessarius Justindo, but maybe that makes it too officer-heavy.
I will eventually have the klickies to create four 20 man centuries, which will combine to be one Legion. I'll attach 5 cavalry and 5 archers to each century, and have a Centurion (red transverse crest & cloak), Optio (white transverse crest & cloak), Tessarius (probably same as Optio), Signifer and Cornicern for each. My reading suggests that each century had more then one Standard bearer, but that would probably make things look too Standard heavy. That'll make 35 klickies, not counting horses, and one wagon for baggage, for each Century.
My Officers of Centurion rank and below will carry a round shield, and cavalry an oval shield and long spear.
My units will be based around 180-200 AD, in Britain, not sure if it'll be besed on an actul known Roman unit, but there's plenty of choices available. I've done one Hadrian's wall story, and when I've dug out the wall sections, a small length of Hadrian's wall (or Londinium's wall) would look good as a background.
Pictures of my Centuries so far - I split my 24 Legionnaires into 2 centuries of 12 for now, as I had enough officers, but they'll mass up to 20 men each when they arrive in a couple of weeks.
I'm not sure if the Centurion should have a spear, pilum, or just his rod of office. The Optios at the back are keeping the men in line with spears, but I could cut the points off to make staffs.
I'll need to make another spares order to get the Red transverse crests for 2 more centurions-