Yeah, the joy and the curse of this is the detail.
In general, if a weapon has a specific nomenclature, like LBX or Ultra, it needs specific ammo. Having Ultra AC/10 Cluster ammo doesn’t help with a standard AC/10, etc.
Same with Streak missiles; you have to pair Streak launchers with Streak ammo. However, stuff like Acid or AoE or FASCAM ammo for missiles can be used with any “normal” launcher. So, FASCAM (scatterable mines) LRM ammo can be used by any LRM launcher (you choose it from the ammo selector in the battle screen for the 'Mech, during combat). If you have it loaded on the 'Mech, of course.
There are several types of artillery you can use. A 'Mech has to have that type of hardpoint though, and they can be rare. The most common is the Sniper Artillery which you can mount on a Shadowhawk; it shoots HE, Inferno, or Shaped Charge rounds with modest AoE and effect. There are Mortars and larger artillery pieces as well, though I have yet to come across a 'Mech that can use them (in my stable at least). Long Tom’s are vehicles, and you can buy them for like $2.2million in some systems.
Generally, as I understand it, pulse lasers are good against fast movers, as they ignore some pips of evasion. Extended range offers more range and damage at the cost of heat. Clan stuff is generally lighter across the board.
Ballistic weapons (and the game is confusing sometimes as to what type of weapon goes where) offer more stability damage than energy weapons as a rule; if you want to knock folks down (and you do) ballistics and missiles are the way to go. Missiles are more reliable because from what I’ve seen your MechWarriors went to the Imperial Stormtrooper School of Marksmanship, with grad work at XCOM Academy. Even high-percentage cannon shots tend to go off into the ether.
There are also some interesting weapons stuck in the category that is usually just MGs and flamers, so that’s worth a look.
The mod also offers a lot of weird variants like Chemical or Binary Lasers, and crude versions of some ammunition types, etc. Pays to read the text when choosing. There are also a lot of single-shot or two-shot weapons that can’t be reloaded in combat. Do high damage though.
The single biggest thing I’ve noticed though is that the mod really wants you to pay attention to your lance’s skills and how they intersect with their loadouts. That, and many of the missions that aren’t hard-wired for four slots like Flashpoints really, really benefit from landing two or three lances, not just one.