I remember reading somewhere that fish eat all kinds of veg and as I am at this very moment preparing some spring onions I wondered if its ok to feed them spring onions
No, nothing from the onion family should be fed to fish.
The best vegetables for guppies are peas. Cook them very lightly (I just sit them in boiling water for as long as it takes to make a cup of tea!) and squeeze the two halves out of the shell
You might want to crush or chop them up a bit as guppies have such small mouths.