Pond liner can be fine as long as you are considering 0.75 or 1mm butyl liner and not upvc as this goes brittle and cracks over time. The fish would not puncture butyl liner. (Although be wary of pacu as they like to chew at creases in the corners so if you want to keep pacu then put a second (cheap upvc) liner inside the butyl one).
Stocking is very much a personal choice and whilst the adult size of the fish should be considered buying very young fish would leave you with an extremely bare pond for a number of years so often fish are grown on in tanks etc and moved in as they grow. Equally if you h as be opportunity to move some to other keepers then you can pick and chose as they grow and off load others. Alternatively maybe accept part grown rescues.
I am not going to mention stocking levels and “rules” as these vary widely and can vary on fish types, filtration, water change regime, decoration and other things.