if you mix normal resin in with filler, it will be harder and the shrinkage will increase.
I have two main polyester based fillers I use, both by Upol (Isopon are the same company, just different brand name).
I use Upol Fantastic and Dolphin glaze. Fantastic is very similar to Easy lite, which is a bit easier to find in stores (e.g. halfords) however you can get Fantastic in a bag/tube instead of a tin, so it stays fresh for longer. Both of these are better than P38 which tends to dry very hard, and therefore is bit of a 'bug-r' to sand.
Dolphin glaze is a stopper, so very thin and runny, however I mix it with a bit of fantastic if I want a filler with a finer consistency.