How common are conformal fuel tanks?
Technically speaking, conformal tanks don't take up hardpoints, so I'm not sure what we have today has an equivalent in older designs.

Otherwise, I'd say somewhat common; streamlined fuel tanks and other conformal packages have been used for quite a while in several different designs. 

Some World War II Supermarine Spitfires used what was called a "slipper tank" - a streamlined package of up to 90 gallons (additional source).

The early 1960s-era English Electric Lightning used a 'conformal ventral store' that could hold either a fuel tank or rocket engine (?!). The Gloster Javelin used a similar set of "bosom tanks".