The level of the Ability
Combined Form does not determine the "level", or how powerful, the form is (which is twice the number of combined characters/vehicles for Abilities and Weaknesses, like how
Transformation and
Vehicle work). That is determined by the number of characters or vehicles that are used for the form. What the level of the Ability does is determine the number of forms is available.
So
Combined Form is a separate Ability from
Transformation and
Vehicle, not a sub-ability, but works similar to how those two abilities work. The difference is the points available for the form is determined by the number of combiners instead of the level of the ability.
Each Dinozord has at least
Combined Form 1, and each Dinozord contributes +1 for the combined form. Since the original Megazord is a combination of five Zords, that'd make the total Vehicle level of the Megazord 5 (or rather, 10 points to spend on Abilities and Weaknesses).
Coincidentally, that'd also mean the combined form will never be Vehicle 1, since you need at least two characters/vehicles to combine.
The Mastodon, Tiger, and Triceratops can combine with the Tyrannosaurus and the Pterodactyl to create the Megazord, but can also combine with the Dragonzord to make the Dragonzord Battle Mode, and the original Megazord can combine with the Dragonzaord to make the Mega Dragonzord.
So the Mastodon, Tiger, and Triceratops has
Combined Form 3 (since they are used in all three combinations), while the Tyrannosaurus, Pterodactyl, and Dragonzord only have
Combined Form 2 (since they are only used in two of the combinations). They'd probably all gain a level in
Combined Form when they get Titanosaurus to make the Ultrazord then.
Since the Dragonzord Battle Mode is made of up four Zords (Mastodon, Tiger, Triceratops, and Dragon), that'd be a combined Vehicle level of +4 (or 8 points to spend on Abilities and Weaknesses). Likewise, since the Mega Dragonzord is made of up six Zords (the original five plus the Dragon), that would be a combined Vehicle level of +6 (or 12 points to spend on Abilities and Weaknesses).
If you are ignoring the Dragonzord and Titansaurus powerups and focus only on the original five, then the five Zords only need
Combined Form 1 to form the Megazord (and since the Megazord is made up of five vehicles, it'd still be equal to Vehicle +5, since 5x1=5)