A few thoughts idea:
Planetary Defence Forces: not sure why you think they'd be less professional? A planetary governor is going to ensure his troops are suitably armed and armoured. I'd leave the Armour personally, maybe try to make them a little less 'organised' by using a mix of weapons, (Maxmini and microart studio both do a range of 'lasguns' which would look great mixed in, same re: maxmini torsos). Use catachan plastics too and mix them up.
Victoria Lamb
http://www.victorialamb.com/store.html#penal has a range of IG suitable head sprues and complete component figures to represent IG from the highlands, (kilts and bagpipes) these could meet your needs as militia:
Penal Legion too, going back to the old RT IG days.
http://pigironblog.blogspot.co.uk/Pig iron Miniatures are about release a range of componment heavy troopers and kolony troopers that would also fit you bill. Add to that the new rebel Kolony Head sprue on Si's blog and again you've got a true mix match which would suit the whole refit and rearm by the weapons merchants/buy what's availble, scavenge the rest kinda look.
Good luck and yes I am an old RT 40k nut
The Commander