Well not to be a smart butt here, but I'm gonna be
You could always go african and get a Altolamprologus calvus or compressiceps ...... they grow slow. Though I don't think they would outgrow a 29gal, Nor do I think they could hang with a JD, but hey they do grow slow. I think I read somewhere that it can take them 3-4 years to reach maturity, note that isn't their full size.