is there a way to only get Monthly Unique Visitors rather than Daily on omniture?
You can select yearly, quarterly, monthly, weekly, daily, hourly unique visitors in Adobe Analytics. See the screen shot below.
Also if you select Unique Visitors the uniqueness is determined by the date range you select.
Unique Visitors is always in respect to your time granularity. So if you trend by day, you will see daily, if you trend by month, you will see monthly.