The warning message about compatibility refers to apps that are 32-bit. You shouldn't get it with Poser, which is 64-bit. I'm running PoserPro on Mojave on an iMac, and everything's fine (apart from the last renders window, the contents of which are in random order). I won't be moving on to Catalina as most of my favourite apps are still 32-bit. And from the posts on MacRumors, Catalina betas are still full of bugs.