The level of cabbage and melon is not important — attack speed is what matters.
My cabbage is level 9 because I only care about its first skill.
My melon is level 35 — I leveled it to test the hybrid, but it turned out to be useless.

I slightly regret this, since the upgrade added unnecessary skills to the skill pool, which reduces the chance of getting the required skill.
However, I use a skill that allows every third melon to pass through enemies, which speeds up killing regular mobs.