...
Is it just an old wife's tale that Vallisneria and Saggitaria or
Cryptocoryne and Vals(?) do not thrive when kept in the same tank?

I've had them in the same tanks for many years, my Crypts and Vals are
nothing but weeds. Serious old wive's tales.

Might stem from soft vs hard water notions of days past.

Regards,
Tom Barr