So it's a possibility of a guppy and a platy interbreeding?
yes, very strange, but has, can, and will happen(most likely)
Is it difficult to breed healthy fry from two beautiful specimens? Waterpond you mentioned dying from supposed imbreeding, how would I avoid unhealthy fry?
The only way to avoid this IMO
is to by show quality guppies(you need money for that). I always buy a male from one LFS
then a female from another(or vice versa). This way you are almost quarenteed to not have hillbilly guppies that reproduce with their cousins, brothers, sisters, or other family members. Make sure you get some healthy looking ones too. I find that the smaller the tail is, the easier to care for(dunno why). Not difficult to breed healthy fry at all.
Answer your questions?