SmackSim Which cruisers do you find "not necessary", thougth there are abilities on some, like the Serevun Overseer's Mobile phase Dection which I find uesless after some tech upgrades. Also do you have any ideals to fix these fillers?
VS Human players I'm sure cruisers can make or break battles. It is (was?) vs. the AI that I think they're a little questionable. IE, in balanced encounters the cruisers can be deciding factors, but when the AI is fielding spam frigates, why research and deploy all the support cruisers? These things take up supply spots and lower overall DPS/supply, so when you have an advantage already (economics, sheer numbers) it becomes less useful than simply overrunning the AI.
I find (pre 1.03) that by the time I research support cruisers, the game is in the bag vs. the AI. In a large map vs 8 hard AI, when I've wiped out one AI neigbor and have the second on the back foot, I know that I'm past the tipping point, so I just want to accelerate the end. Losing 10-15 frigates because I don't have Hoshikos, Overseers, Subverters, etc.. is no big deal. I will always take heavy cruisers though, because they require even less micro for the end-game. Just send a blob of 20 into a system and check back in a few minutes

.
I like Guardians too. Advent really use them well.
Cruisers I think will be better with 1.03, as the AI will actually use them now, so countering will be worthwhile.
Also do you have any ideals to fix these fillers?
More diversity in infrastructure, resources, ship types, and strategic branches? Less impact of Trade Ports by themselves: first weaken them significantly so that taxes can be a larger part of the overall credit-base. Rework this so that research avenues open up a variety of economic possibilities: Unlockable planet upgrades that impact taxes, so that a player can make money without relying on trade ports.
How about researching planet-type specific credit income other than population? Then players could choose to focus on certain types of planets for their tax base, and this would actually be a sensible option?
Look at the X series of games for economic ideas. No need to get as complicated, but even adding 1 more resource to a game like this, and adding infrastructre around it, could improve the tech tree and the 4x side of the game. How about a refinery that combines metal and crystal and turns out a new resource? A resource that isn't required for ships, but for special abilities?
How about troop ships for capturing enemy ships and planets?
How about ships without jump drives, like hanger-based fighters, but of any small variety? Cheap system-bound ships would present a viable defensive option and leave players with tough choices. Could be a 2-level drive system with the second level being capable of inter-stellar jumps. To jump non-stellar ships they either need this drive or need to load onto carrier ships. (then increase stars/planets ratio for most maps).
The tech tree is just a little too simple at this point to support the 4x aspect of the game. Its an excellent RTS tree though! Some of the abilities and techs in the game are better than any other game out there.