fleet prefire chambers? i don't think those exist, even if they did, they suck compared to tac consoles that buff a particular weapon type (disruptor coils, phaser relays, ...)
yes all tac buff cosnole stack without diminishing returns.
fleet rcs certainly can make your ship turn better, but when you are at a point where you want 4 of those, i believe you might not want to fly a cruiser in the first place. One is OK, with 2 you already need to sacrifice important +dmg consoles (zero point, assimilated, tachokynetic?,) or other important consoles (2 set console for the d'deridex)
also the rcs have no diminishing return, but buff only your base turnrate...so even with 2 rcs with 40% increase you are looking at a possible 2 - 3 degree turnrate increase. at the cost of valuable console space. imho definately not worth it.
even though d'deridex can use DHC, it's questionable to do so.
They never changed it. It's been the same since the big skill revamp (season 5 I think?), and the only difference before that was that the beam and cannon consoles gave the same boost as the energy type ones, with the penalty for mixing energy types being built into the skill system.
Tax consoles stack and it is very much recommended you go full in with the energy type of your choice. Meaning if you use 3 antiproton and 1 quantum, use full mag regulators (the anti proton tac console) and no quantum tac consoles for best results
Rcs consoles do stack but with diminishing returns...say you have a +40%, a +38% and a +36% set of consoles equipped. The +40% gives you the full benefit to your base turn, the +38% gives you half for 19% and the +36% only give a quarter for 9%.
I am under the impression that the game calculates the console with the most turn rate first, then applies the half diminishing return to the second highest and so on...
2 is already a shaky proposition and beyond that is a crap shoot. Best to look at skills and trying to maintain a higher impulse speed as speed grants turn rate,