Both Java moss and fern ship well. They are low light, classed as easy to keep. Just don't bury the root crone, just the roots. I don't know that it would be dense enough to protect your baby guppies from the parents. You would need to plant several quite close together. I raise my baby guppies with the parents and use Hygrophilia Difformis(water wisteria) and Myriophylium planted densly together. They are low/medium light stem plants and pretty fast growers. Luck, OS.