Hello,
Regarding your first issue, this is a bug in 5.5 version. Fortunately, it has been resolved by 5.5.2 hotfix. Please try to apply
latest one (the higher, the better).
Concerning your second tagging query, yes, this would involve customizing tag selector form control. You know, when you have custom table, it is not connected with particular document and therefore you will need to get current (or any other) node from code-behind and obtain document tags from it and pass them to your tag selector (and vice versa).
Best regards
Ondrej Vasil