Small invertebrates (not shrimp) would be the best bet. 13 convicts in a 29 gallon will be a very crowded tank very soon.
If you have fewer convicts I would suggest some small semi-aggressive loaches as they are about the only non-cichlids that are tough enough to live with convicts. Botia eos, Botia morleti, Botia almorhae, or Botia modesta. Just don't make the mistake of picking up a Botia Hymenophysa...that little devil will probably kill the convicts off over the course of a few months.