Playing a shadow sorcerer for the first time, about to get to level 3.... Seems to me quickened spell would allow me to use my action, for example, to cast Mind Sliver - where hopefully they fail the save. Then use my BA to cast something like charm person that they'll have to subtract a d4 from the saving throw for. Kosher?

You have to be acutely aware of the Bonus Action spellcasting rule when using Quickened Spell Metamagic.

Bonus Action (https://www.dndbeyond.com/sources/basic-rules/spellcasting#BonusAction)
A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven't already taken a bonus action this turn. You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action.

So - if you cast a spell - any spell - for your bonus action (whether it is quickened or innately a bonus action) - then you can only cast cantrips that turn that use an action. If you cast a levelled spell as your action then you cannot use a bonus action spell.

Right, mind sliver is a cantrip - so that would work? Basically I'm just making sure that I can use this in the order I need it, to set up my enemy for failure on the quickened spell. 

In this case - yes - because it doesn't break the BA casting rule. It has to be Action Mind Sliver and then Quickened Charm Person. The other way around would break the BA casting rule.

Excellent, thanks!

Quote from Emmber >>

In this case - yes - because it doesn't break the BA casting rule. It has to be Action Mind Sliver and then Quickened Charm Person. The other way around would break the BA casting rule.

 To be clearer about this, you can quicken before casting a cantrip. Action/bonus action order doesn't matter.

But you can't quicken the cantrip then cast a leveled action spell (or quicken after casting an action spell).