There is a risk associated with the too early introduction of solid foods (including just a little bit of rice cereal in a bottle). These included an increased risk for food alergies, diabetes, and obesity.
In general, it's best to wait until your baby is at least 6 months of age before starting solid foods (although many doctors still reccomend age 4 months).
Here is a link about adding cereal to the bottle:
http://www.babycenter.com/expert/baby/babyfeeding/9155.html
Here is a page with various links and information about the right time to start solids:
http://www.kellymom.com/nutrition/solids/delay-solids.html
Note: This website is generally pro-breastfeeding and Attachment Parenting, but the sources linked are generally from widely recognized, mainstream medical sources (American Academy of Pediatrics, World Health Org., etc.).