The choice of metabolism type (warm or cold blooded) depends on the activity type of your creatures. Endothermic (warm blooded) creatures tend to consume dozens of times more energy than ectothermic (cold blooded) creatures and are also far more active.
If you want your carnivores (meat eaters, hunters) to be active predators and need to hunt a lot, then you have to make them warm blooded. A python or boa constrictor eats only once in 3 months or so and then goes on a long sleep. Other snakes, lizards and alligators (all cold blooded reptiles) eat far less than mammals which are warm blooded and far more active in their environments.
This gets all the more important when your carnivores are large sized (10 feet or more). Such creatures would need a lot of warming up before they could get active on a cold winter morning. You don't see snakes out in the wild during winter as the temperature is too low for them to be active at all. In contrast, wolves, lions and other mammalian predators are active year round because of their warm blooded metabolism.
The summary of all this is that if you want active, agile predators that are often hungry (need to hunt at least three times per week), then you should go with a warm blooded metabolism for them.
Now comes the issue of herbivores. Here you have more choice. Some herbivores (take fore example, elephant sized ones) are so large they don't need to run to protect themselves from predators. Due to their low energy requirements (lazy lifestyle), these can afford to be cold blooded. Smaller herbivores (anything that needs to run to protect itself against predators) would definitely have to be a warm blooded creature.
For big, huge, giant animals like Sauropod Dinosaurs, things get a bit complicated. They are so huge that the fat layer(s) on their muscles protects their inner organs from cooling down even when the temperature is too less. This shielding effect provides them with a sort of semi-warmblooded metabolism which is known as gigantothermy.
Giant carnivorous dinosaurs such as T-Rex, Giganotosaurus, Spinosaurus, Carnotaurus etc are also thought to be semi-warmblooded if not completely endothermic.
Another important factor determining warm or cold blooded metabolism is how fast you want your creatures to reach adulthood. For example, for creatures such Bruhathkyosaurus, scientists estimate that if they were purely cold blooded, their hatchlings would require more than 100 years to reach adulthood. However, if their bodies incorporated any type of warmbloodedness or semi-warmbloodedness (e.g. gigantothermy), then the hatchlings could reach adulthood in ~35 years. Now that's some serious difference!
A purely warmblooded creature requires nearly 4 times the food that a cold blooded creature of the same type and weight requires. Elephants spend most of their time in feeding. Now think about a sauropod dinosaur and imagine how much it would need to eat, if it were a purely warmblooded creature? Also consider how much vegetation is available in the region. A group of warmblooded sauropods, the size of Seismosaurus, Diplodocus or Amphicoelias would literally strip an area (of 2-3 miles) clean of all vegetation in less than a week. Also consider how much dung they would spread in the region due to their fast digestive systems. I guess you don't want to have that in your environment. Shit happens
If you have active carnivores, they have to be warmblooded, no matter what their size.
If your 4-legged mammal-like creatures need to run to protect themselves, they should be warmblooded.
All herbivores which are just too large to be in the scope of any predator, should be placed under gigantothermy (basically cold blooded, but so large that their internal body temperature remains nearly the same because of thick fat layers).
You CAN have creatures which switch between cold and warm blooded metabolisms as they grow from infancy/hatchling to adulthood. If you have elephant sized coldblooded/gigantotherm creatures, I suggest you keep them warm blooded in their juvenile years and then gradually switch them to coldblooded as they reach adulthood. This would simply mean that they produce less heat from their food. Although no examples exist in our world, it is theoretically possible. This is specially important if you don't want to put your creatures in juvenile times for dozens of years (like the sauropod dinosaurs).