Why not do something retro and base it on classic Mac OS apps? Each one has a defined minimum and preferred memory allocation (reserves its memory at launch and has to stay within that allocation).

Otherwise, if you’re using a modern OS, most processes will allocate memory as they need it, depending on use. If there isn’t enough available, background processes will be dumped to virtual memory to make room for the active process. If all memory is used up, the active process will start moving not-recently-used data onto storage as well.

Classic Mac OS lines up much better with your scenario, and is easier to populate as you can just go on infinite mac, boot an OS version, load up some applications and get info on them to see how mich they allocate.