When I try to swap roms between internal and cm-10.2, multiROM says there isn't enough free space. When I had 3 different roms installed, it reported that it needed 2500MB but there were 2400 free. I removed one rom, now it says I need 2924 but there is 2904 free. Is this a bug?
No, it's how the partitioning in the phone works. Delete all ROMs except the one you're using and see if there's enough space to install another one. (Or install TWRP or CWM and you can change ROMs more easily.) Android phones aren't designed to hold multiple ROMs in storage at one time.