Two Choices:
Plain IG Codex, but mix things up...
Or, if your gaming group/tournament group (and it is a rare tournament that does) allows Imperial Armour Armies, then the Siege of Vraks (1,2&3) is the best bet.
Vraks 1 (IA5) has just plain Renegade Guardsmen
Vraks 2 (IA6) adds in a Khornate Flavour, with some mutants and other fun debasity.
Vrake 4 (IA7 - my favourite) goes the furthest to Nurgleise everything, with lots of putricity!
As long as each thing looks like each thing (These Beastmen Gors with Lasguns are Guardsmen, these Ungors with Lasguns are Conscripts) keeps things well in theme.
Ogryns very easy become "Spitting Chaos Spawn".
Want Commissars? Why not Beastmen Champions with special weapons and kit to reflect? You've already got Banners for Standards, and hell, Musicians can be Vox-Casters...
The biggest problem with "an army of counts as" as that Counts-AS is rarely constant and rational across the board.
Hell, Straken is a Chaos Marine with a Darkblade by any other name, as far as i'm concerned.
Be Consistant. Be Constant. Be Rational, and let your inner gribbly go!
(This messagebrought to you by a proud owner of 3 Renegade Guard armies, only 2 of them chaos based).