[non] monstrumve submisere Colchi
maius Echioniaeve Thebae
Colchi and
Echoniaeve Thebae are both nominative and parallel in construction, joined by the conjunction
-ve; the name of the city
Thebae, "Thebes" is always plural (as it is, at least in form, in English), like
Athenae, "Athens",
Mycenae, etc.
"nor did the Colchians or Echonian Thebes bring forth a greater monster"
The epithet Echoniae is derived from Echion, one of the warriors who sprang up when Cadmus sowed the dragon's teeth, who married Cadmus' daughter, and so is one of the founding fathers of Thebes.
Colchi is a reference to the Jason and Medea story. Jason sowed the dragon's teeth and a band of warriors sprang up and killed one another off.
https://en.wikipedia.org/wiki/Jason
In both myths, a race of fierce warriors springs from the teeth of a slain dragon sown by the hero.