it depends on the theme you want to hold.
If it was for me, i'd say this:
Since this is a Penal Colony Army, the chances that a Leman russ was stationed there are utterly low. Any kind of heavy tank, in fact is almost impossible to find where there is no strong imperial guard presence. A planetary defence force rarely has many of those beasts.
A penal colony relies on its fortifications, Arbites personnel and, of course, the prisoners themselves to defend.
The sentinels are perfect, I can see them being used as recon squads to hunt fugitives and to patrol perimeters. Chimeras are ubiquitous, the devil dogs are based on the chimera chassis, so a totally plausible choice, as the hydra and medusa, that can simply be chimeras with cannons taken from the fortifications. Anyway the real theme of the army should be infantry. A big penal planet can house millions of prisoners. I imagine that even if the planet goes rogue and the prisoners are given a certain degree of freedom (becoming the lowest citizenship, while the former governor and penal officers would rule as nobles) they would still remain "slaves" (as in many imperial world). There would still remain a large portion of the carceray population deemed too dangerous to release, and the prison itself would still remain fully operative (there are always rioters and political enemyes to chain to a wall). This is my take in expanding the fluff and justifying my following list choices, from what you have said before
About the list: obviously I would want a funny and fluffy list, without being an aut-lose list. So (mind you, I'm not writing with my codex on my lap, so i can't give point values, i'm just writing freehanded
-Company Command Squad
Astropath, 2 bodyguards
The governor of the planet with his retinue of veteran guards (armed with old but effective laser cutters, taken from the prison workshop), and his Astropath.
The two guards are brutes recruited from the most loyal and dangerous ex-prisoners
A powerful former-prisoner, now free to be the hidden power behind the scenes. Everyone in the prison fears him, and everyone has to respect his power and authority. A dangerous enemy, but a preciuos ally too.
Those are the old Arbites veteran teams, grizzled warriors, sporting the deadliest weapons of the armory. They are deployed in their heavy anti-riot combat suits, via Vendetta's Grav-Chutes
5 Infantry squads (to blob or unblob depending on the battle)
5 granade launchers
5 heavy bolters OR Autocannons
3 heavy weapons teams
The spine of the army is comprised by ex-prisoners, armed with all the planet can produce en masse.
3 Grenade Launchers
2 Infantry squads
2 grenade Launchers
2 Missile launchers
The former Warden of the prison now leads his Arbites in daring manouvres always in the thick of the fight!
He is the dangerous one. Better to give him a target....
3 Scout Sentinels
The hunters, the fast and deadly mechanized fist of the Governor. Once used to scour the outskirts of the prison, now they support the prisoners against any enemy!
Many big guns have been stripped from abandoned or ruined parts of the prison walls, and have been fitted on the adaptable Chimera chassis. The Manticore, in particular, is armed with modified rockets, with warheads taken from the very Cobra Freighters that patrol open space around the planet!
here it is. I guess i've mantained the same point level. About the Aegis, i've just noticed that you have two Hydra and 2 vendettas. You should be ok. if you feel still exposed, then, remove an infantry squad from and a commissar from Platoon A) and you can afford the Aegis w/quad gun
EDIT: I am a fool! How could I forgot the true fluffy part! I should add this:
2 Penal Legion Squads
The prisoners might have been freed, but not everyone. These are the most deadiest, lunatics and dangerous. Be they maniacs, heretics or demented, they still wear the explosive necklace they used to wear when the prison was still functioning. Just to be shure....