FormerlyHemlock
Hero
As someone who's less concerned about resource attrition then I am about alpha-strikes and novas, this doesn't bother me. The one combination I might worry about is Sorcerer/Other Caster. I would probably say that only Sorcerer slots can be turned into Sorcery Points, and vice versa. I would allow metamagic to be used on either spell list, though, or the combination isn't really worthwhile (and much less fun!).
Yes, I agree--the prima facie obvious ruling is that only Sorcerer slots would be able to be turned into Sorcery Points; and for a Paladin, only Paladin slots can be expended as smites.
Unlike you, I would probably even rule that Sorcerer metamagic only works on sorcerer spells. Seems reasonable from an in-world perspective: sorcerer magic is different than wizard magic and can be manipulated in different ways.
So you wind up with a scenario in which a multiclassed spellcaster is clearly inferior in all spellcasting-related ways except endurance to a single-classed spellcaster. If you want to do crazy tricks like Quicken Eldritch Blast while laying down an Empowered Meteor Swarm, you have to do PHB-style multiclassing; old-school multiclassing isn't going to get you there.