For example, creating a Sorcerer 3/ Druid 2 character, who (as a level 5 multiclass character of entirely spell-casting classes) has four Lv1 spellslots, three Lv2 SSs and two Lv3 SSs. A level 3 sorcerer knows four sorcerer spells and would have four Lv1 SSs and two Lv2 SSs, so I would pick my sorcerer spells based on this. However, does this mean I can only learn two Lv2 sorcerer spells? Or can I learn three Lv2 sorcerer spells and a Lv1 sorcerer spell? Could I even just learn four Lv2 sorcerer spells if the Lv1 spells aren't appealing to me, even though I wouldn't be able to cast them all yet?
I understand you can only pick spells from a class spell list from as high a level as you have spellslots from that class (eg As Barbarian 11/ Sorcerer 9, you could pick sorcerer spells up to level 5), but not much else. The PHB just says you can learn class spells "of a level for which you have spell slots"