[@Rondo of Blood] For multiclassing, I personally go by these rules;
You must qualify for all of your stat requirements. If you are a cleric wanting to dip into rogue, you must have 13 WIS and 13 DEX.

You must reach a minimum level of 3 in order to multiclass once. If you want to multiclass again, you need to have a minimum level of 6 (combined, so it could be cleric 4/rogue 2).

I'd also like to advise you that if you are planning on doing a spellcaster/martial build, don't go with Barbarian. Barbarians cannot cast spells whilst raging, and since that's something you want to be doing as often as possible, maybe stay away from that. (Also stops you from needing to spread yourself too thin - Barbarians need STR, DEX and CON, and Warlocks need CHA. A Fighter just needs STR or DEX.)

[@Vexxed Vex] We're still open, yeah! The current party consists of a kobold storm sorcerer, a kenku rogue/eventual arcane trickster, an aasimar monk (in my own personal headcanon, they're hard to distinguish from humans), and potentially a tabaxi war cleric, though the player in question may be changing that.