I remember when someone asked me about; #Adenium / #desert-rose (or any soft trunk) could be bonsai? YES! it is! a lot of sample Adenium #bonsai in google. This concludes that everything could be bonsai, but please let us imagine first, are they like the other big trees? If its look like the big tree just make it to small! But you must have some tricks to create from soft trunk to be look like the hard woody trunk part. Here's my suggestion #bonsai-advice :
2. Cutting/Pruning; Cutting branch must careful to see the buds along the cutting, please cut the branch above the branch buds growth. And the remaining branch cuts must coated by fungicide liquid to heal them. The rest of the cutting part might be face upwards to speed healing and accelerate the branch growth. After cutting process you must need to be wait and be patient, my suggestion don't watering until 1 weeks or less than that, to avoid bacteria and fungus in plants.
3. This is the tricky part, cause the trunk of Adenium is so soft and we must draw pattern from the root to up, this magically looks the old trunk like and hard woody like. Please watch this out --> https://youtu.be/LKHPtgWDWU8 (im sorry please ignore the language hehe its Indonesian version just watch it.) I hope it will help you guys! Have fun!