Well, why do you need more types of ships. Perhaps there aren't any more, maybe it isn't the lack of information. The UNSC suffers in space combat for a reason, and are at a SERIOUS disadvantage in almost every advantage. Why not make them unique from the Covenant and not give them both analog ships of each other. The Covenant could have many more types of ships (or whatever canon says) which are more powerful, but cost a lot, and the UNSC could have less types which are cheaper, or whatever. The UNSC started winning battles because of creative and open-minded (and daring) warfare. Their technology, numbers, and money all were MUCH worse. But, as the Covenant were very narrow-minded, they tended to fight by-the-book (remember the whole Cortana controlling a Covenant ship and using Forerunner technology in a more efficient and creative way, but getting criticized by the Covenant AI for doing so?). UNSC is willing to try anything and do anything to win - the Covenant do not leave their shells of no-creativity and innovation.
That's just my take on it - make the UNSC have better crews and more capabilities, perhaps "last stand" type things, but make the Covenant have a serious advantage in more types of ships, more firepower and armament, and more money. I don't ever understand the obsession over balance and analogs between factions - balance isn't realistic, nor is it the most fun - someone is always the underdog, and winning as the underdog shows true skill (loosing as the "overdog" shows poor skill, LOL).
But, as I stated before, this isn't my mod. I just thought to throw out some constructive criticism - I feel I have the right to because of a big history of modding/mapping, where I was always constructively criticized or given ideas (or just bashed).