Greetings.
Allow me to offer some insight on building a small stand-alone force that will also work quite well as an ally for another Imperial army.
For starters, let's look at your
HQ choice.
You'll likely want to run a Company Command Squad, or if you're looking to keep it somewhat cheaper, then I'd recommend looking at a Lord Commissar. Of the two, I'd suggest running a
CCS though.
Company Command Squad offers a Company Commander that can issue orders, and has 4 guys that all have
BS:4. This is the prime place to put either a heavy weapon, or several special weapons that you want to hit regularly when they can fire.
I'd recommend the following build for your
HQ:
Company Commander w/ a Bolter (It's a super cheap upgrade, and worth the points, plus if you go with Victoria Miniatures models to make your
CCS, you could easily convert the Company Commander to have a bolter.)
One Veteran w/ a Lasgun and a Voxcaster (Again, cheap upgrade that will help tremendously when you're throwing orders out)
One Veteran w/ Medipack (Somewhat expensive, but gives Feel no Pain for the unit, helpful when you take small arms fire, and also helpful for the 'Gets Hot!" wounds you'll take.)
Two Veterans w/ Plasma guns (Again, pricey, but you want these on the
BS:4 models and it's always fun when you can throw the ignores cover order on plasma guns.

)
Lastly, buy this unit Krak Grenades (Not for the
HQ model, as he has a bolter, but Krak Grenades allows that lone Lasgun Voxcaster model to chuck a krak grenade if they're within 8" of the target instead of firing the Lasgun, one more Higher
ST: shot is always groovy in my book)
Total point cost for this squad: 115 points
Next, your troops choices:
First, a veteran squad will fill out troops choice 1 nicely, riding in a chimera.
Veteran Squad, Sergeant with a Bolter, squad wide Krak Grenades, 6 lasguns, 2 Plasma Guns, and one Grenade Launcher (or Flamer, your choice), and one Voxcaster in here with a Chimera Transport will run you 176 points total for the squad and transport tank.
I suggest running a Multilaser in the turret and a heavy flamer in the hull for utility's sake (I get a lot of mileage out of my chimeras which are built with this weapons configuration). The reason I suggest running 2 plasma guns and one grenade launcher/flamer (your choice) in this squad is that only two models can fire out of the back hatch of the tank, but not putting a third special in there seems like a waste. Keeps the squad somewhat fluffy too with the Mordian predilection toward grenade launchers. The other option you can consider for this squad is a pair of meltaguns instead of plasma guns, which will take their cost down to 156 points. Again, use your
BS:4 troopers for hard-hitting weapons.
Your second troops choice can be a standard Infantry Platoon, consisting of a Platoon Command Squad, and two to three Infantry Squads.
Platoon Command Squad:
Platoon Commander w/ a bolter and a
CCW, 4x Guardsmen, one armed with a Lasgun and a Voxcaster and the other 3 with Grenade Launchers. Keeps the squad nice and cheap (51 points), and the grenade launchers go the same range as both the Jr. Officer's bolter and the Voxcaster's Lasgun. I wouldn't spring for the Krak Grenades on this squad, as they're all only
BS:3 here. 51 Points for the
PCS.
Lastly, your Infantry Squads. I'd do 'carbon copies' of these squads as follows:
Sergeant, armed with a Bolter once again, backed up by a Grenade Launcher special weapon and a Lascannon Heavy Weapon. These are your line-holder squads. I suggest you typically run them with the combined squad rule in place so you have a mass of 20 to 30 (minimum 20) guys with 2 (or 3) lascannons and grenade launchers. One point I will make, and this is to save points, the first of these squads, purchase with a Voxcaster, and keep that model in the back so you don't lose it early. The rest of the squads you want to run without a voxcaster, to save the points. So long as you're running them with the combined squads rule, you only need one voxcaster to benefit from the re-roll, so buying a voxcaster for each infantry squad is a waste unless you plan on running each infantry squad as it's own entity (not reccomended).
With the voxcaster in the squad, each of these squads runs you 81 points. Without the voxcaster, the squad runs you 76 points.
Running the Platoon Command Squad above, backed up by two infantry squads (One with a voxcaster, and one without a voxcaster, but armed the same otherwise, I.E.: Bolter/
CCW sergeant, Grenade Launcher, Lascannon Heavy Weapon Team) along with the Veteran Squad and the Company Command Squad, you're right at 499 points exactly, which is right where you said you wanted the list to be for a stand-alone force. That gives you 40 infantry models and one transport tank at 500 points.
You can add two or three more Platoon Infantry Squads or even Special Weapon/Heavy Weapon squads to the Platoon to get up to 750 points or if you prefer armor, throw a Leman Russ or a couple of Support tanks into the mix (Manticore, or Wyvern would be my suggestion).
That's my suggestion for your list. Check my math and make sure I didn't miss anything on the points values.
It's pretty fluffy, has some decent shooting teeth, and you can issue up to 3 orders per turn. Two from the Company Commander and one from the Jr. Officer in the Platoon Command Squad.
Don't forget also, if you are issuing orders, the orders MUST be the first thing you do in your shooting phase, no matter what, even if the guard are allies to your main detachment. Always start every shooting phase with your orders, to include tank commanders orders as well.
That's all I've got for now. Best of luck and welcome to the guard

Glad to see someone else fielding a 'non standard' regiment. I personally field Mechanized Vostroyans, so seeing some Mordians represented on here as well would be awesome.
Hope to see some pictures once you get the models and get them painted.
Take it easy.
-Red__Thirst-