Here are the 4 Fs you will need to ensure for any creature: Fight, Flight, Feeding, Fu- Reproduction. Although in this case, fight and flight are not too important and those are details you should be able to flesh out yourself.
Well, the first big problem with large creatures is that they will need food. Lots of it. Gargantuan herbivores would have to consume large swathes of vegetation to be able to feed themselves, so they should not be in large packs. Gargantuan carnivores would be a bigger problem, as they not only need to eat a lot of meat, the animals the carnivores eat will also need to eat as well. This means that there either can not be too many of these large creatures, or you will have to introduce more food into the foodchain.
The quickest and simplest way to solve the lack of food is to introduce gigantic plants as well, so your gigantic herbivores can eat them, and then the giant carnivores can eat them. If you do not want quite that many large plants around, and instead want it to be slightly more "fictional", where dragons can swoop down at their leisure and pick off cows, where giants can pick up deers for a quick bite, one funny thing you could then do to increase the number such low level foods for the giant creatures is that you could make them able to do photosynthesis, so there can be large numbers of them, and they would not devastate the nice green fields of the fields.
Another way you could feed your giant creatures is via chemotropy. In real life, most things that use chemotropy are very tiny, and live in dark places like underwater volcanoes. In this fantasy setting, they could literally take boulders off mountains and snack on them, and when they die, they fall over and calcify, turning to stone or something like that.
Many fantastic setting like to depict giant creatures to also have a huge lifespan, as well as not reproduce all too fast, this may become a bit of an issue for you as there is an element of human over hunting that you believe should be an issue. Let me note here that I am going to assume that none of your large fantastic creatures are physical gods, that they can be slain by mortal men.
As such, you will probably want them to breed much faster than they normally would, given their size and age. In real life, the reason why large creatures tend not to breed so often is actually because of the lack of food. If they try to breed like rabbits, then they would not have enough food to feed their young. (No references, sorry) But if you solve their nutrition problem as stated above, they would then reasonably want to breed as fast as they have food to supply their calorific needs.
This kind of situation was similar to what happened to the human race. Once agricultural methods were developed, and there was a large supply of food, the population will then bloom. So actually it is very dependent on the energy available to them, almost like modern human society, where much depends on energy
Now, for cohabitation. The nature of the food chain is that it is more accurately a food web, as things can be eaten by multiple different things, as well as eat multiple different things. So, what this means is that all food chains tend to move to one central, apex predator, which sits at the very top. The problem now is that you want cohabitation, which I will take to mean that there are multiple such predators, that have no natural predators of their own.
The biggest problem then would be they would then be competing for more or less the same resources, so you can either make the resources so endless that there is no need for competition, or maybe you could also make all these creatures live in vastly different biomes, so there is not that much competition. Another thing you could do might be to create an actual apex predator, that is capable of snacking on dragons, griffins and ogres alike. This may then encourage those large creatures to respect one another in fear of a common bigger enemy.