It might be too late now, since you're already part way through, but an army themed around the ragged remains of a grand army could be great. Have each squad painted up as a different chapter, but cover the guys in battle damage - and perhaps the odd piece of armour the wrong colour, to represent scavenging. Maybe even the odd piece of non-standard weaponry, like an imperial guard grenade launcher rather than a bolter, to emphasise the ragtag band theme.
Otherwise, for your army, I don't think that most casual players would complain - but unless this sort of thing is a carefully thought out theme, I can't see it looking that great.
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