Aircraft Performance Question
 

Can an A320 or B737NG descend at 210kts IAS clean? At any rate of descent?

umm is this a serious question? they can descend at 210kts: if you remove the thrust then you've either got that or stall. most airlines look unfavourably on pilots who choose the latter

just as an example if you were 210kts then you'd generally need flap 5 to hold that speed doing 750'-800' per min on a standard 3 degree glide, obviously depending on weight and ambient conditions

but to be honest i don't really get your question. you'd definitely struggle to get any high rate of descent clean at 210kts

Weight, altitude and configuration?

Yes they can, but not clean at high altitude/weight.