Hi,
Once you've played it thru you will begin to know where what units are. In this case I would replace vampires and griffins for this reason. You will most likely not find more for quite a while. I'm guessing you are using devilfishes and marauders because of the bonus that Jimmy Kraud gives them? In my opionion both are quite crappy. A solid set-up for the first 3 islands or so is for me often inquisitors, royal snakes, royal thorns, priests and perhaps bowmen. This helps you to get some achievements faster too. (burning, poison etc)Keep searching them treasures and with a little luck you'll get the wandererspell call colossus for a random level 5 unit! Once you discover continent of verona you will
get alot more fun choices of units. Anyways. Those units mentioned you will be able to buy almost unlimited. Except the royal thorns though. You will find only 4 I think but even if your leadership allows like 7 eventually they are probably still worth keeping until you find a level 4-5 unit. Anyways, this is just a nice way to start out. Its not optimal but it would allow you to keep the same sound army from level 1 to 10 I'd say. Have fun playing, I know it's awesome!