i'm missing a fast unit(lancer or something)

possibly some kind of bomber(probably a bird) flying over enemy units dealing minor damage.
regarding some units, you might have to restrict which ones are available for each level, for example a cloudy level wouldn't allow for flying units(Plants vs Zombies uses this system, you could play through it to see how it works out)

you could make a gladiator a dueler/harvester, aka if/when he kills a unit your resource goes up(faster)

you could make a pirate capture enemies and let them fight for you

I've decided for the pirate to use bombs. He drops one every few seconds or so. enemies that run into the bomb activate it and cause an explosion of AoE damage. I can have special gear for him to increase the rate at which he drops bombs, too.

The Gladiator I still don't know what I'm going to do...

The Gladiator I still don't know what I'm going to do...

How about throwing a net to bind an enemy for a few seconds.