I've never found any allelopathic interaction evidence in aquariums
with 300 species of plants.
Most every aquatic botanist I know agrees that an alleopathic effect
is very minor at best. There might be special cases in natural
systems perhaps, but generally, not. In aquariums the likelyhood is
even much less so.

Plants do fine in both hard and soft water. A few such as Eustralis
and R. wallichii do seem to do better at lower KH's but they do grow
in harder water. These are the only two exceptions I can say may
exist. The same plants can be found in hard water and soft water only
a few miles apart.

Regards,
Tom Barr