[ammulder]

If you're a starting human with Impaired Strength 3 (I know, who would do that?) and you have, say, a Charisma of 6...  can you have a Strength of 3 (because 3 is lower than the "maximum for the selected metatype"), or can you only have a Strength of 2, because Charisma is maxed and the "maximum for [Strength] decreased" to 3 and you can only have one attribute at its max?

I guess I'm wondering if "for the selected metatype" is significant wording...

[Stainless Steel Devil Rat]

If you lower your max Strength to 3 and then have Strength at 3, yes that's your 1 attribute allowed to be at racial maximum.

[Xenon]

... or can you only have a Strength of 2, because Charisma is maxed and the "maximum for [Strength] decreased" to 3 and you can only have one attribute at its max?

This.

I guess I'm wondering if "for the selected metatype" is significant wording...

The selected metatype is still important.
For example, a Dwarf with Impaired Strength 3 that have Charisma 6 would be allowed to get Strength 3 during chargen.

Also, note that a Human with Impaired Strength 3 and Exceptional Charisma that have Charisma 6 would also be allowed to get Strength 3 during chargen.