Gozantis will be really good at both carrying fighters and killing enemy fighters. ISDs are only really super effective at killing large targets, so spamming them probably isn't a great idea. Carracks are good at taking out small ships, as well as supporting larger ships in engagements. Same with arquitens and dreadnoughts. Lancers are end game, but great anti-fighter. You will want a good mix of ships with different roles (you can see their roles in their descriptions, as well as what they are armed with) My recommendation would be no more than 3 large capital ships, some carrier support of any kind, frigates for fire support and enough pickets/hunters to cover your important ships. The latter are especially important because both your main enemies have powerful fighters/corvettes/gunships. For early game taking out corvettes, tie bombers with their missiles are great. Thats about it really. Its just balancing the roles of your ships with the gaps you need to fill. Capital spam no longer works

In the beginning I had to learn that with v2.9 Turbo Lasers nearly can't hit smaller targets any more and have difficulties hitting frigates. So no capital spam only.

That means - as mentioned above by MessyPaladin - you always should bring a good mix of types into battle. In Normal-difficulty I had one fleet with 1/2x ISD (maybe Ozeel + Vader) + Venator + VSD + 5x Gozanti (Bomber loadout) + 2/3x Arquitens, + 1/2x Dreadnaughts + 2x Carracks.

If you haven't done so already, it's worth taking time to read all of the orange guide text in the beginning of a campaign about the different classifications. It will be there for the whole game when you click the "X" where you check for missions.
All corvettes are not the same. Hunters are best seeking out smaller ships, screeners are for taking out enemy fighters.
Superiority fighters are dogfighters, Interceptors are best at eliminating bombers. Strike craft can do a bit of everything.
Some capital ships are destroyers, some are monitors, some are torpedo boats. They all do different things better than the other designations.
It's a LOT to read, but knowing those guides better will help you assemble an effective fleet.
Also, as with any strategy game, scout the enemy, a LOT. If the rebels have a lot of hunters (Hammerheads, CR 90's) They're going to shred your smaller craft fast. If they have a lot of bombers/strike craft, your bigger ships are in trouble.