Ok, so I know that at the start of the game you only have access to Phasers and Disruptors, and that's both good and fair in my mind, and that as you get promoted you have access to newer and better weapon types, again, both good and fair, but I can not find a list anywhere that lists what is available to you at your rank. What do we get access to at each rank? Just wondering.
It's generally tied to the tier that you could start skilling for them in the old skill system. So at captain you start seeing plasma, tetryon, and transphasics drop, and at admiral you see the rest.
Several of them start dropping a mk level below their rank - I know plasmas drop rarely at mk IV even though you couldn't skill for them until mk V, and of course there's special vendors and mission rewards that are outside the progression, so it's all very fuzzy, but they generally don't seem to have changed the drop system along with the skill system, so the above is approximately true.
On a side note: The later available weapon types aren't "better". The only difference between e.g. Phaser and Polaron of the same mark and rarity are the specific procs. The polaron weapon could trigger a reduction on ship power levels, while phasers can knock one random subsystem completely offline.