With main type, need complete events for make that monster avaible ingame. Maybe this's possible unlock all main type monster with events, I'm not sure, I didn't try get all monster before
With sub type, most in them just need make combine monsters having. Some rare monsters only have from other disk. Example Kasumi:
http://monster-rancher.wikia.com/wiki/Kasumi
This multi disk feature still follow game rule, only create what monster already avaible. That's why I can't pick Golem (not avaible at beginning), the monster I guess it can do 1 hit KO to all AI
About my monster, it's possible to get wihout need any other disk, but it need very long time to setup to get. Of course I can make TAS with default monsters having ingame, but I don't want later have someone will say "Boo! i know other monster can attack faster you"