Prototype RL (-1 cluster) already exist for succession wars play. Of note, they are more expensive and have that cluster penalty compared to their production models that came along later. I would apply the same to the MRM... I would say its possible, but as a bespoke missile weapon (not in mass production) they cost 10x as much and take that -1 cluster penalty same as rocket launchers. If you wanted to go as far to say that MRMs are in production, then you would need to make lots and lots of MRM succession wars mechs with MRMs to explain it, which is too much work honestly to try and brute force MRMs into your setting, especially when rocket launchers exist.
As a side note, if you DID allow MRMs in otherwise lvl1 play, I dont think you will be adding much to your setting. As stated, no variants exist that carry them so the random enemies wont have them to loot for salvage, and without fancy pilots or targeting tech the +1 penalty will rightfully scare players away from making it a primary weapon. An MRM 40 with 2 heat sinks replacing an AC20 on something like a hunchback seems ok but hitting with the ac20 was already hard enough, so I doubt the extra 6 hexes of range will matter much as you wont be able to hit those shots with the +1.
Now, rocket launchers are a different animal. Having a few rocket launchers on a mech isnt as much of an investment, no ammo to explode which makes a ton of sense in succession wars play, and several of the rocket launcher variants already exist that are good for 3025 play as the RL is the only advanced weapon. Really, just not exploding is enough of a reason to make the swap on mechs like the assassin and jenner and panther. An SRM 4 for 2 RL20s is a nice trade, letting you clear mines too, and multiple mechs have that random srm4.
