Can I buy the Aero cap separate?

No, they are only sold as a pair only

Do these caps fit on the older Aero wheels?

Yes, they must be used together. Aero cap on its own will not fit with the older sports cap

Do they come with wheel badges?

Yes, they come with HDT centre badge

Do they come with wheel nuts?

No

What size Aeros do these caps fit?

These will fit with the 15″ and 16″ Aeros