Few questions:

can filler be put over paint (ie por-15)?

what is the best filler to use (easiest)?

can i paint POR over filler or does the POR have to be put down first then POR over that?

Filler's cannot be applied over paint because of the lack of adhesion. Filler is best applied to bare prepped metal. Some glazes allow for minimal spot fixes over primers and that is "MINIMAL".

POR produces their own filler's & putties for use in conjunction with their products but these are very expensive, NEW and I have not used them as yet.

POR can be applied over filler but it will not have the same durability as it would if applied direct to metal.

I use Evercoat Products (http://www.evercoat.com/productSection.aspx) exclusively and highly recomend their use.

Evercoat Rage Gold Filler
Evercoat Metal Glaze
Evercoat EverGlass (Short strand fiber glass)
Evercoat Metal-2-metal (metal reinforced)

Check out their website below.
http://www.evercoat.com/productSection.aspx

Hope it helps and answer's your questions.

Most fillers these days can be put over "preped" paint in good conditon. The bond the original paint has on the metal is strong. Body shops do it all the time, they dont have time to take every repair down to metal. It is an excepted method of repair and will last a long time.
I would not do it on paint that is old, faded, cracked....So it depends on the paint condition.
Sand the POR with 80-100 grit or whatever POR recomends, you will be fine.