Which is why I suggest having a way of determining your available fleet units. While a navy may well want all of its units to be largely the same, the reality is that a facility to build naval units (expecially *big* units) is a serious investment in both materials and skilled personnel (and skilled personnel are usually the sticking point) and they take time to come online and build up. This naturally limits their ability to deliver a never ending stream of heavy units. What's a Navy to do? Wait 12 months for a slipway capable of handling a BB or CV, OR punch out 3 FF's or a pair of DD's and at least be able to get something into space that you can use for some kind of defense.... Once you have gone through this decision process you have the units to deploy as you will in any fleet composition you like (presumably informed by your strategic situation if relevant).
Depending on the socio-political structure (or maybe even the species if you go for non-humans) underpinning the navy some of these decision points can be adjusted, but the basic points remain valid. You need to be able to get the right people in the right numbers together in the right facility with the right resources to be able to build a naval unit. Which units you build are determined by a whole slew of factors such as your ability to keep the facilities supplied with resources and staff, how much time you have, how your general population feels about supporting a massive navy, external factors such as what your enemies are doing and how they are doing it.
Drawing an analogy to to an airforce is, I think, tempting but incorrect. For starters aircraft are fairly small and fragile (which is why the airforce buys in bulk) and although complex, building one is a very different engineering challenge to build a ship that is huge and can take multiple hits. In this respect modern ship building is the closest existing model for building large spaceships. Sure, the high-tech required for a ship negates some of the issues around building large structures, but then you are building something that is vastly more complex than a ship for a wet navy, so it balances out. Also, if a navy somehow had only one or two kinds of unit then those units would have to be capable of performing every task the navy requires and therefore unless the ship is truly enormous (and even then) it's unlikely to be able to do everything well. Even in combat related matters, your enemies will be trying to find tactics that will work against you and only having one or two designs makes that job quite bit easier for them, because your available set of "good" tactical and strategic choices becomes very well defined and fairly inflexible.
The only way you might get anything an endless cookie cutter set of vessels is if your resources are so vast that you effectively have an unending (re)supply of units. In this scenario the only real challenge you would face is an equally vast opponent, sufficent combined oppoents that you cannot muster enough local superiority to crush each individual opponent or massive internal dissent that essentially break your forces in half.
Hmmm... another ramble. Hopefully useful though.