Yes, I would increase the water hardness for some of those mentioned fish, particularly the livebearers as they will not do well long-term in acidic or soft water. The easiest and safest way is the natural way; dolomite, crushed marble, crushed coral, are the usual. Dolomite is the best of those because it adds both calcium and magnesium, the two main mineral salts causing hardness ["salts" not to be confused with sodium (table salt), quite another thing]. A small quantity in the filter if possible; or mixed in the substrate, but this takes some experimenting to avoid turning the tank into water so hard it is liquid rock.
The rainbow are slightly basic and medium hard water fish. Melanotaenia boesemani
is said (as it notes in the profile) to not do well in soft water, so this species should be in with the livebearers.
The other fish including tetra, barb, will fare better in 6.5. So you should separate the fish accordingly which would provide ideal conditions for all with none having to "make do."