Deanna - I would probably just make it as per the instructions on your custard powder packet. It is okay to use normal fat milk as long asit is cooked. You could try using formula/bm but not sure how this would go. also not sure about freezing - once again, I think you just hve to give it a go. :?
Sorry not much help after I read this, but I have made a sweet couscous dessert for James using cows milk and have frozen it. It worked well.
Hope this helps
Lisa mum to James my textbook/spirited koala