@Tijn
I mostly agree with your selection of armies for the 4 classes. Peasants are useless, of course, and Dwarfs/Hydra are much more useful as castle defense creatures than in a traveling army.
The one class where I’m not conclusive on is Barbarian. Ogres are very tough but are also slow. So, choosing between them and Goblins, I would sometimes prefer to take Goblins with me for traveling (as the hero moves faster). This is especially true if you are in a map with lots of Goblin huts where you can get more Goblins to join you for free. So, sometimes I would choose Ogres, sometimes Goblins.
In the end, it does not matter as much as one thinks, because at some point in the game you will have more than one hero, and you will have conquered some castles of other classes. In the end your most powerful hero will probably have a mix of powerful creatures from 2-3 classes.
Dragons, well, they are totally awesome. They are way overpowered in this game. It is true that their speed is Medium, so a fast creature (like a Phoenix) will get to attack first, but it typically will not make a difference. Their total spell immunity allows massive exploits – take a strong spellcaster with a few Dragons, walk around and cast Armageddon / Storm, causing hundreds of damage points to all enemy creatures, while the dragons are not affected.
In the sequel, they tried to balance things out a bit, by adding ‘Dragon slayer’ spell which boost attack against dragons, and adding powerful Titan units that, while not immune to spells, can also benefit from good spells (like Bless), and can often match and surpass dragons, depending on the skills and spells available to your hero.
