Way in the beginning, in episodes 1/2 he implied he didnt choose what the things would turn into, but that changed as quicky as the reflect damage shenanigan...
I think he needs something with a similar size to grow animals, but vines/trees can be grown out of anything, there has to have some hidden rule or common sense because for as much bullshit as Giorno can pull, it's never insane as the example you gave xD but yeah the only constant seems to be:animals have to be the same size of source material and the size of each wound has some saying on how quick they'll heal, and maybe giorno's condition factors in too
I have an issue with edits too so i get the feel D: yeah that fish was huge if it needed to pull two people with it