PuraSleep Serenity Mattress - 5 Sizes

Can these be used on top of bx springs?

Yes!

how does the Serenity mattress compare to the Coolflow in terms of comfort and thickness?

CoolFlow: Comfort Level - Medium firm (5.5 to 6 on a scale of 1 to 10)
Serenity: Comfort level: Medium Firmness (4 to 5 on a scale of 1 to 10 with 10 being the firmest)