@Aedman
If you are already training cerebus (training ground is unlocked in Ch2) and have some frogs from other buildings (since you say they are 'stuck'), your issue is probably not noticing the tabs at the top of the window where you pick your units for the battlefield (at least that was my issue when I was learning combat early on!). There are 3 tabs at the top with similar crossed-swords icons. The first is default (shows up after clicking 'fight' from the main encounter screen). If you click the second, it will open the training grounds and you can select cerebus from there. The third is the merc camp ( not unlocked to build until Ch7) and you can select frogs from there. If you craft Vallorian Valor or Grounds of the Orc Strategist buildings in the Magic Academy, you can get those units well before you unlock them in-game as well. Vallorian Guards will appear in the merc camp tab and Orc Strats in the training grounds tab.