Just to give you a very realistic update on your question.
The rental market is VERY tight in Longmont right now. There are some owners that will allow small pets - however they typically charge an extra pet deposit as well as some additional pet rent. You may want to broaden your 'acceptable' cities criteria to include the Frederick/Firestone/Erie/Dacono/Berthoud areas to increase your chances of finding what you are looking for.
Hope this helps!