I used for a while 1 GB here too, but they are absolutely useless with MOS. 256 are really enough, 512 better, but really not needed for MOS/AmigaOS software. Maybe instead more Ram can be usefull for Linux/MacOnLinux.
Moreover what you say has no sense, MorphOS supports how much Ram you'll install, the pobelm you encountered is probably an incompatible iussue between the Pegasos hardware and your Ram modules. Try other kind of Ram.