Part of the reason is that they seem to trade weapon technology quite actively amongst each other, so if 1 starts researching a certain weapon type, you can almost expect the other races to soon follow suit.

I've had cases where the AI would add defenses specifically targeting my weapon type but that may only be when they are either at war with you or planning to do so soon.

It's much better for you if all of them do focus on the same weapon type; that way you only have to research one defense line, and your warships are good vs all opponents, instead of having upgrade or change designs for different fronts. 

I haven't had the Thalans as much of a threat recently.  Can you override that preference by dumping missile tech on the Thlans, or do they persist on using Lasers when they've been handed Harpoon? 

I think thy dummied down the Korath between Dark Avatar and Twilight of the Arnor

I think every 50 turns or something close to that the AI are supposed to re-evaluate their ships based on their enemies ships. I have no idea when this is calculated or if it only depends upon your ship design. I know that I usually just buy their ships in most games and build constructors so I don't notice much

Oh, I've seen the AI switch weapons occasionally. One DA game had everyone using either missiles or beams, and using defenses against them as a result - which gave the Krynn a big advantage when they suddenly started using mass drivers, even though it set them back. In the TOA game I'm writing an AAR for, both the Korx and the Altarians had low-tech mass drivers, but when I traded them Laser V, they switched to that and subsequent beam weapons - and then the Altarians started going back to mass drivers when their enemies' shield technology got too good.

So it does happen. I've never seen the AI using multiple weapons at the same time, but I don't do that much myself, so I can't say there's anything wrong with that.