Just thought I'd let you all know the results of this...
In the week leading up to the holiday I could only find lots of 3-400 goldfish fry around 1-2cm, too small for what I wanted. I went to Jansens when they were doing their 'buy two get one free' thing and picked up 30 neons and 30 lemon tetras (they were the best value for money in terms of size, thier small goldfish were around $6 each! The guy gave me a bit of a discount too).
On friday night I did water changes on both tanks, and gave the fish a big feed. Saturday morning I dumped the 30 neons in the 3' tank with the small ornates and dats, and the 30 lemon tetras in the 4' with the big dat and big ornate. The big dat got about 4 in one mouthful as soon as they hit the water, then looked content and ignored them, as did the ornate who had a bulging stomach full of beef heart from the night before. The two small ornates were also fat and well-fed, but still went crazy chasing the neons around. My sisters BF came around the next morning, and phoned me to say the two ornates looked MASSIVE and all the neons were gone!! There were still a few lemon tetras left, but most of them had gone too.
I returned today to find the smallest clown loach in the 4' tank missing, and two others with scrapes on their heads, most likely from the big ornate.
Conclusion: Fail. Needed more and larger feeders for the big fish (someone really needs to start mass-breeding cheap plain goldfish!!) and the smaller fish could probably have done with larger feeders too.
I'll post a pic of the bloated ornates later.