Here are some excellent links my friend.. you know as well as I do that Roman law wasn’t laid out in some singular book called Roman Law, but changed and developed over the centuries according to the whims of the leaders.
http://nostalgia.wikipedia.org/wiki/Polygamy
http://www.wikinfo.org/index.php/Polygamy
http://www.wikinfo.org/index.php/Polygamy
These links don’t say anything about any Roman law against polygamy for their subjects.
The whims of the leaders were of course written down; this is the only way a procurator in Judea, for example, could possibly know what the whims of the emperor were.