You'll usually want a heavy cruiser and definitely two Emeralds. Beyond that there's some choice.
>Amber
Basically a budget Onyx for people who already have a different heavy cruiser and don't want another. Solid brawler.
>Granite
Garbage. You want a particle lance, take an Obsidian, Ruby or Diamond.
>Turquoise
Split CAW/bombardment ship. Basically just wants to get close and brawl/bomb, but can afford to be a little more sneaky about it than gun ships since there's no need for weapons free. Blue balls may not seem that great at either of their jobs at first, but that 12" scan range really helps out. Work extra well in pairs or with a Jet, that way they can compensate for their lack of bombardment power by attacking the same target from far away.
>Basalt
Solid carrier. Does carrier things and has decent guns, you can protect your shielded dudes from CAW/bombers with fighters or use your own bombers as a very versatile weapon. Wouldn't take more than two and probably not even more than one, due to the pricing issue.
>Onyx
Pure brawler, made to get stuck in and go weapons free. 12 3+ lock attacks is vicious. Remember to put up those shields.
>Obsidian
More of a sniper than a brawler, but still tough. Can put out nasty firepower against heavily armoured or shielded targets, and not as reliant on weapons free to do damage.
>Jet
Turquoise that brawls better. More focused on combat than bombardment compared to the Turquoise, but can still help out. Doesn't need weapons free to be effective like Onyx does, but can't put out quite the same damage either.
>Azurite
Eh
>Aquamarine
Eeeeeeh
The light cruisers aren't great. Emeralds are always worth the upgrade from Azurites because they offer so much more with troops, and Aquamarines bank on an unreliable and situational gimmick.
As for frigates, you can make 4 Opals, 4 Amethysts and 4 Topazes and not be left wanting. The rest you can do whatever you like with.