I love this tree, and I think I can give you some information, but the problem is going to be the size of the diorama. If you make it in any of the scales that the Schleich or other brand animals come in the tree will be just under 9 feet tall! The tree gets to be up to 200 ft tall, in scale that is about 100 inches or 8.3 feet. That is a big diorama!
To answer your questions, the first thing you should know is the subspecies confusion. There is only one species, and no subspecies. The reason for the different varieties and subspecies designation is the tree has been transplanted to West Africa and tropical Asia (mostly India) and when it is in the non native countries they give it their own name, translating into varieties and subspecies which are not scientifically valid as far as your diorama is concerned. There are several different growth patterns as well, and this is more important to accuracy then the name, if the tree is growing in primary rainforest it grows straight up and an umbrella shaped canopy develops above the rest of the forest canopy. Look at photos of South American rainforest and the trees that stick up above the rest of the forest and then spread their canopy are probably Kapok trees. If it is growing in secondary forest, or has nothing restricting the light it's first branches are found closer to the ground and it does not get as tall as one in the forested areas. Also there is a different look to the age of the tree, younger trees grow thorns on the trunk to protect it from climbing animals and anything that wants to eat the bark or tree, but as the tree grows these do not grow with it and as the tree gets to be large enough they stop producing them so in an adult tree you have few, short spines while in a younger tree you may get a medium amount of thorns that are fairly long and sharp and as a juvenile there are lots of sharp thorns to protect it as it gets large and strong.
To make your diorama, there are some time saving tricks I have learned, to make the leaves get the two smallest sized daisy flower paper punches you can find. Different sizes gives the tree more realism. These will be the leaves and remember, the leaves of this tree are only found at the top. Do not try to put the flowers or pea pod like fruit and seeds on, they only produce these in certain years and they do this when they have dropped all of the leaves so it would be a better model to have the foliage and not the flowers/fruit. I can and will give you a great tutorial that I have written for a modelling club on how to make trees if you want, let me know and I'll send you to the link or email you a copy with the photos and instructions. There are also many tricks to making the frame and the bulk of the tree that not only save time but are very easy to work with and make an excellent bark and trunk material.
Finally your habitat question. If you type in a search such as what animals eat kapok trees or animals that live in kapok trees you will get a brief list of the animals found on or around the trees. Easier still is to just pick the country you want the tree to have come from for the diorama and just search for native plants and animals of that region. Central American rainforests have much different species as Peruvian rainforests so just pick the country and look up birds, mammals and native plants from that area and you have the beginnings of your species list. Same with the plants, they are host to a full range of epiphytes that live there including bromiliads, mosses and vines. Along with those come different frogs and birds that rely on the water stored in the overlapping leaf structures that collect and hold water. Again, decide what country you want the tree to come from and pick plants and animals that come from that location, if it climbs or lives in trees it will probably be found in the kapok tree. Things like birds including toucans, king vultures, eagles all are big enough for the diorama and live in the tree. Mammals such as monkeys, ocelots, jaguars, and any of the possum and raccoon types of animals can be included in the diorama, they all live in the canopy and can be added to the branches. Other things like dart frogs and tiny tree dwelling rats and birds will not be easy to get in the scale, if your tree is large scale, the diorama will be prohibitive to make, and if it is smaller the small figures wont be able to be sculpted or purchased. Try dollhouse stores for certain things like small birds or mammals, they make some really well made smaller bird and rodent sculptures but the scale is either 1:12 or 1:24, the smaller the less detail you end up with but the fact that you might not have the space to make the tree in full scale you may have to give up detail for a reasonable model and diorama.