Cold SadHungry is right on the common weapon type, keep them all as one type.
Volkite can be pretty brutal because you don't expect it to be.
Four shots per model, 2 up to hit re-rolling ones, 2 up to wound most things re-rolling ones and extra deflagrate hits which wound on a 2 up re-rollable will do a number on most infantry units.
The unit of irradiation engine myrmidons absolutely has to be in a triaros to survive long enough to shoot.
I originally thought i would love conversion beamer squads, but i haven't really enjoyed using them - i used to have a lot more fun with them when we could only take them singly on a techmarine, hiding them in places like a sniper with a wound sponge of servitors picking off stuff from across the board. good times!
I'm in the process of painting up some photon thruster myrmidons and i think these will scratch that long-range low-
AP itch better.
In fact, i'm wondering if the photon thruster might actually buck the logic of taking the same weapon throughout a squad - if you have a photon thruster accompanying a group of volkite or irradiation engine myrmidons then you could do a number on marine units which rely on the sarge in artificer armour at the front!