Mixed armor sets don't really become a thing until hardmode since armor pieces in prehardmode are usually quite homogeneous (all necro pieces have 5% ranged damage, all meteor has 9% magic etc.). There can be some armor mixing in early hardmode but it reall starts once get acess to the DD2 armor sets, these armor sets have very lopsided stats that you can take advantage of.
With any class/summon hybrid build your head slot Usually doesn't involve summons, there are so many good headpieces do to pre plantera armor sets often having 'your helmet is your class' attitudes.
The best head piece for mage is hallowed as it has the best offensive stats and a nice 100 mana the reduces how often you have to replenish mana.
With ranger you want to go shroomite although at first they may seem weak an limited compared to pre plantera head pieces their damage is multiplicative with other damage bonuses, so if you already have say 50% ranged damage adding a shroomite head piece is like adding 22.5% ranged damage and 5% crit. If you're using a weapon that doesn't fit any of the helmet bonuses go with adamantite, and if you want to save ammo cholorphyte is there too I guess.
For melee you want to use either chlorophyte or shinobi. Melee speed generally isn't that useful of a stat as only a few good weapons have dps that scales off of it. Chlorophyte has the best raw offensive melee stats. Altough you shoul consider swithing to shinobi it has slightly weaker melee stats for much higher summon stats.
With chest pieces there is usually a summon focus unless you are doing ranged.
The spooky chestplate has +2 minion slots which is a massive bonus that whole sets get built around
Forbidden robes are a good option before you get access to spooky.
If you want survivability you can instead opt for valhala even after the nerf its regen bonus is still really strong.
With ranger you will generally progress from frozen->huntress->shroomite->red riding. Ranger has the best chest pieces so they're really the only class that will opt out of the spooky chestplate post plantera.
With melee you may consider using the monk's shirt, but you'll usually swap it out latter. Mage lacks good chest pieces.
With legs well this is sort of the reverse of chest pieces.
Rangers don't really have good pants, mythril greaves are nice early on, but this will usually be spider, tiki or spooky.
The dd2 leg pieces are absolutely dominant when it comes to melee and mage leg options with 20-25% crit and almost as much summon damage that goes well with the +2 summons on your spooky chestplate.
Forbidden treads might also be worth considering for mages as the got 10% magic damage in 1.4.1, while also adding a minion.