Without spolers please.
Just started to play and I wonder, which would You suggest to do?
Create custom members of group and get all 6 slots asap or wait and "hire" characters on the way?

P.S. If there is mechanic that kills all my custom made members because I moved to other zone or something let me know. And once again, please keep it spoiler free.

If you want to engage characters in their personal 'quests' (not really quests), then the way to go is choosing the built-in characters. If you want to play the game your own way, without other characters getting in the way, feel free to create your own. It's ultimately up to how you want to play the game.

There IS a mechanic that kills all your followers shortly after the beginning of the game, so you must wait until you get to the first town after escaping the tutorial dungeon, get to an inn and create new followers there. What happens is when you escape the dungeon, the area you escape to is scripted to kill everybody except your own character because it's a tutorial dungeon, and the first two characters are tutorial characters only.

The base game gives you the most essential classes for your party members. DLC gives you three more decent additions for good measure. So the game gives you enough class party combos to work with.

Custom characters don't get their own story, obviously. So not only can you miss out on optional story bits but also expee. However, companion story progression will trigger based on your level (though some quest stages cannot be pursued until certain points in the game) and you can shelve any companions you have at your stronghold, whether they're the ones the game gives you or hirelings you've made yourself. so you can dust off the built in companions long enough to do their quests, and then go back to adventuring with your custom built party of hirelings.

Hirelings are also silent. They will not interject during dialog scenes so you can lose that flavor while adventuring with a party of hirelings. Hirelings also don't banter, so you'll miss out on dialog exchanges between companions.

Hirelings also cost money and higher level hirelings cost more to make. You can choose to make lower level hirelings at lower cost, but hirelings will *always* be one level less than you. The built in companions are free and on par with your character's level. You will not have enough gold to purchase a full party of top level hirelings at the start of the game.

The benefit to adventuring with hirelings is:
1.) you can create your own fun custom party. Party full of mages? Barbarian's only? Rangers, Party of 12? This is best for fun second+ playthroughs.
2.) you can fill a position in your party with a class the game doesn't give you two of (which is all of them). I like having two mages in my party, so I often hire a mage. I also don't like Durance, so I hire my own cleric.

The game never kills companions and you can never lose them, so don't worry about it. Well, the two you get in the prologue are scripted but that's irrelevant since you can't get hirelings until afterwards anyway.

EDIT: any companion or hireling can be perma killed if their health zeroes out, but that's on you.