I'll ask for some advice: Most of my CAs are >= lvl55 now, so I'm looking to level some of my CLs and DDs
Which ships should I level? Or should I continue leveling my CAs instead
The general idea is to have decently leveled ships on all classes, it's usually suggested to have at least 2 of each class so that you have a good chance at an event. I'll give you a rundown of a bunch of the basic idea of the main ship classes used. I'll be ignoring the ones that aren't easy to obtain or can't be obtained though construction or drops.
Battleships are always good to have since they're your main source of firepower
Practically all the BBs are worth it to level up. the BBVs are earlier to remodel and are needed in their own cases like composition for events and when you want to hit 1-5. Nagato and Mutsu are a solid pick for when you just need a BB as they have high firepower, AA, armor and HP. The Kongou sisters are FBB which is good when you need a fully fast fleet while carrying the power of a BB and is usually favored by everyone.
Standard Aircraft Carriers, they're like battleships but they use planes and they're always Fast.
Like Battleships, all of them are good. Akagi and Kaga both have 1 equipment slot with a high plane count while having a high plane count, they also have the highest HP and second highest armor. Souryuu and Hiryuu start off with low stats but stands out when the turn Kai Ni, they gain a plane redistribution like no other CV and gain a substantial boost to their firepower and Anti-Air stat. Shoukaku and Zuikaku have good HP, the second highest evasion(Hiryuu being first), and a large plane count that's distributed almost evenly. They're a good choice for when you need a large amount of planes like having one of them full of Reppus just to gain Air Superiority. Taihou is a special Carrier, she not only has high firepower, AA, armor and plane count but she has the ability to launch planes at moderate damage.
Light Aircraft Carriers are your aircraft carriers that can target submarines. Though they're named Light Aircraft Carriers, not all of them are Fast, about half of them are slow. They generally have lower plane count than your standard Aircraft Carriers and generally lower stats than them too. I'd recommend leveling up the Junyou & Hiyou for their overall plane count and Seaplane Tenders Chitose & Chiyoda if you need fast CVLs since they generally have one of the highest stats and their remodels carry the Type-A Ko-Hyoteki which are needed by the Torpedo Cruisers.
Heavy Cruisers are pretty much your do everything ship. They shoot torps and deal good amount of cannon damage, they can use recon planes and have 4 equipment slots
Being the jack of all trades, these ships do a lot but unfortunately suffer a lot from it too. When you need firepower, you look towards the BBs since they can kill targets the fastest by dealing large amounts of damage in shelling phase using their high-caliber cannons. Then there's also the CL and DD which performed better with their torpedo stats which were higher than CAs. Though with the recent influx of Kai Ni's in the CA department, they've been placed back are pretty good to have.
Light Cruisers, they throw good torpedoes, deal decent damage in shelling phase and can target subs. There's really not much to look at them when you just need to clear general maps but event compositions would usually require them every now and then. They're pretty much a better Destroyer that has access to more equipment.
Torpedo Cruisers, they don't count as Light Cruisers to remember that when referring to fleet compositions. These girls get an absurd boost to their torpedo stat and gain the ability to use the opening torpedo phase when equipped with a Type-A Ko-Hyoteki(or as people would call it, mini subs). They can normally take out an enemy when they fire their torpedoes but like regular Light Cruisers, they don't really have much firepower going for them in shelling phase and you'd probably gear them towards full on torpedo set for maximum torpedo damage.
Destroyers, they've got the highest evasion stat but generally have the lowest stats overall but you need them. It's usually a matter of fleet composition at times and certain Destroyers outshine a lot of other girls in the larger ship categories which makes them a better pick. It's pretty much all in their Kai Ni forms that have them being standing out. Oh and they're your best expedition girls.
Click here for a list of notable Destroyers
But as I've mentioned in the first line,
you generally need leveled ships in all classes. Where you focus on is up to you, you'll eventually get around to leveling everyone when you've either played long enough or crazy enough.