no, there is no dependency/problem-with-post-edit-buttonierarchy between usergroups… they are distinct entities… users can belong to mulitple usergroups…
a usergroup is just a way to give a set of users a common set of permissions…
so say you wanted to give guests read access to forum X… you would edit forum X and apply the guests usergroup with the read only permission set…. this would allow any guest to only read the forum…
now lets say you have a general usergroup for any registered members called members… and you wanted to allow members to have standard access to forum X… you would edit forum X and apply the members usergroup with the standard access permission set… now members would be able to create topics and posts…
now you want to have a set of moderators on forum X… so you would add the desired users you want to have moderator access to moderator usergroup… you would edit forum X and apply the moderator usergroup with the moderator permissions… now those select users you put in the moderators usergroup would have moderator permissions on forum X…
now what happens if a user is a member of members and moderators? easy – the best permission wins out… the permissions are essentially OR’red together… if either permission set grants a permission, the user gets it… that is sort of the hierarchy you referred too…
of course, you can have an unlimited number of usergroups and permission sets… we have supplied a few default ones for you to use – and for most users that is enough… but, for example, if you wanted to have a different set of moderators for each forum, you would just have to create a unique moderator usergroup for each forum and put the desired users in each…
on the forum – forums – manage usergroups and forums, you can pretty easily see the usergroups/permission sets applied to each forum…
also, when you create or edit a group, you can set up a default set of permissions that will get applied to each forum you create within that group… those settings only apply to new forums… any current forums will need to be manually set up… as mentioned though, there is a tool for creating a new permissions and applying it to all forums within a group…