Ah, your pH could of resulted in your neon and gourami deaths as they prefer soft water. Also, your water conditions could of changed since your water test. Something as simple as overfeeding your fish can bring the ammonia levels up in a matter of minutes in an unstable tank. Also, you should of bought a group of gouramis, a lone one will get quite bored in a tank my himself, and the only thing he can do is chase the neons. Buying a group of at least 4 will reuce the aggression problem.
The platy could well be trying to breed, but I don't think we'll stress you're fish out too much if he is, it's probably the water that killed your fish, not the chasing, but then again every lil' detail adds up. 
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If you have a big enough tank with enough hiding places, pH of around 7, you can keep virtually any fish together as long as all the fish are around the same size and these two groups of fish are avioded:
Serrasalmus
Tetradon(figure eights and dwarfs are the
exception).
I keep a successful community of fish in a 4 foot tank including the following families:
Cichlids, tetras, loaches, gouramis, barbs, rainbows, livebearers, killiefish, catfish, puffers.
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