Hello!
First off, welcome to
40k!

A couple quick housekeeping things, then ill give some suggestions.
When posting up your army lists, only include the upgrades. This makes reading the lesat a LOT easier, and easier to provide help. For example, on your one tactical marine squad you have listed their boltguns, power armour, grenades etc. I will provide an example based on one of your squads
10 man
tac squad
-plasma gun
-Missile Launcher w/flakk
-2x lightning claw on sarge
Hope that helps!
Next up, some list suggestions. First thing is I would drop all the upgrades on the seargents. Their main purpose is to take the attacks of enemy
IC's in challenges to keep them from slaughtering the rest of your squad. And if you drop all of your upgrades, you can get a librarian!
Librarians are psykers, with the ability to take special powers. These do various things (see
BRB), but
DA have access to the Divination tree. This is extremely powerful, as the ability to provide rerolls almost guaranteed is SUPER powerful. Best of all, a level two Libby only costs 100 points!
Next major note will be on the Devastators. A super versatile unit now that most things can split fire, one of your main advantages as a
DA player is access to (relatively) cheap plasma. Rerollable plasma cannon shots (with the aforementioned Librarian in the unit) make almost anything in the game cry, and you can split fire your
ML's at hordes with Frag or at vehicles with Krak. At 750 pts, you wont be running into any sort of
AV 14 vehicles, so the
ML's will serve just fine. (If he does take a landraider he wont have points to take any guys, so you can just walk across the board to his objective and win.)
Small note for the next one, I would drop the flamer and give the man another plasma gun. Flamers are all good fun, great against charging units particularly, but adding in more plasma really lets you threaten anything. The twin gun/cannon combo has saved my hide sooooo many times! The
ML is also a good, but I would drop the Flakk. You wont be running into that many fliers at this points level, and they will provide the same problem to your opponent as the above mentioned landraider. ST7 can hurt vehicles as well, and
AP 2 is nice against any
TDA the enemy has. Or pesky Tau battlesuits....
Next up, Company Master!
I love this guy. Seriously, Monster Slayer seems like sooo much fun, but I havent tried it
IRl. I run Belial with a metric tonne of termies for fun, and Ravenwing for competetive, but its on my to do list! I will give you a couple options for speccing him out, and you can take the suggestions or run him as is. Should be fun!
One thing I would absolutely do is give him artificer armour instead of
TDA. Strikes at intiative, same armour save and invuln save, and can chase things down. All for cheaper. All so much winning! Only reason to make him a termie is if you are deepstriking him with other termies, whic harent present in the list.
Company Master #1 AKA Peanut Butter Plasma Time
-Artificer Armour
-Lions Roar
-Power Axe
Total: 145 pts.
This guys is here to be an all around 2+ armour save beater. A plasma cannon that you can move and shoot (!), and then the ability to have AP2 in close combat is good enough to deal with ANY unit at 750 pts you will come across. Couple that with a squad of marines, and that plasma gun they have, and its good times!
Company Master #2 AKA Marine Captain Masher
-Artificer Armour
-Mace of Redemption
Total: 140 Pts.
This guys takes anyone in power armour (looking at you Vanilla Marines...) chews 'em up, crunches them, mashes them, and spits them out. +3 strength,
ap 3, blind, concussive and by George it strikes at initiative!

Wounding on 2's, and them not being able to hit you back next turn on a failed test is SUPER good!
Company Master #3 AKA Monstrous Creature Eater. Aragorn Style.
-Artificer Armour
-Monster Slayer of Caliban
-Plasma Pistol
This is my edition of your above Master. He is tailored this way to grab a Wraithlord or Carnifex by the family jewels and start kicking. It very much relies on you getting the 5 or 6 to power up the blade though. Instant death is great! Most of those
MC's only have a 3+, so that should be fine, and the plasma pistol is there so you can shoot before charging and hopefully take a wound off. That greatly increases your chances of actually nobbling it if you DONT get your uber blade that turn.
Company Master #4 Thunder Smash
-Artificer Armour
-Thunderhammer
-Storm Shield
This guy is here to fight challenges. He will strike last, yes, but he can ignore most anything with a 2+, 3++. He then smashes face. Pretty straight forward, but doesnt bring any flexibility, and is only useful in close combat. So in a shootout he is just an extra save at the front of the unit. Not my favourite :p
Doing the maths on my suggestions, taking company master #1, it should come out too 715 pts. This gives you a whole 35 points to play with, as well as (
IMHO) a more streamlined and flexible list! YAY! With those 35 points, my recommendation is a Powerfield Generator on the Librarian. A 4+ invulnerable on the whole unit lets them ignore the majority of the fire coming their way that their power armour cant deal with, and they should be far enough away to not get in close combat. It also lets you ignore a lot of flyer attacks, should your opponent be crazy enough to get one at 750 pts.
This leaves the list at 745 points! Thank you for reading this much (if you made it here) and my apologies if it got a bit beardy. I get excited when it comes to theoryhammering on new lists with interesting options. If you have any questions, or want any clarifications on wording, rules or suggestions, please ask away!
I hope this helps, and I am sure you will be taking the heads of traitors in the name of
El'Johnson soon!
Automatically Appended Next Post: Oh, and a note on Combat Squading. This is what your friend was talking about with the splitting into two 5 man units. It is a viable strategy, but I would only do it at the 1000 point and up level. Simple reason being, a 5 man unit is waaaay easier to wipe off the board on its own, and will overall decrease your survivability. Having one team to assault with and one to sit back with the heavy weapon is great in theory, but often just gets the assault squad killed.
Why do we do it then? Simple answer: Rhinos and Razorbacks. Having them in a tin can that can move almost half the board at flat out (usually) and then pop smoke for a cover save is great for getting that assault unit in range! Best part, if the Rhino dies, it will very likely not wound the unit inside. This delivers your 5 man squad with their Company Master of Face Bashing straight into the fight, and ready to dish out pain without taking losses. This requires points for a rhino though, and thats not really happening with the Devastators in the list I'm afraid.
This is the first thing I would consider doing on a 1000 pt upgrade though! Once you have mulled over the 750 pt. list, you can either leave a comment asking for a 1000 pt suggestion or you could
PM me. Im happy to help!