Encouraging your pokémon to work out their differences peacefully is, at the risk of offering fantastically unhelpful advice, perhaps the best policy here. Start by trying to mediate their conversations and offering compromises to their differences. Point out ways that their abilities complement each other (Charizard’s flying abilities versus Feraligatr’s swiftness in the water) and offer fair solutions for whatever it is that triggered their competition in the first place. (If they were competing with one another to determine who gets to sleep in a certain spot, for example, suggest that they take turns sleeping in that particular spot.)
Additionally, encouraging them to compete tasks together off the field could potentially strengthen the bonds between them. Have them perform day-to-day tasks such as chores or certain exercises together. Always train them together, and be firm and consistent if you see hints of competitive behaviors. Give them clear goals that they can only complete when working together, rather than ones that develop their strengths separately.
In other words, cooperation can very much be a trained behavior, but you’ll need to be very consistent and firm about teaching them how to interact with one another. That and it may take quite a bit of patience and a lot of sternness on your part to keep them from simply going at it every time you try to teach them cooperation.
Best of luck!