Tank looks pretty long. Suppose you could get 15-18 if the tank is longer than a meter long, you can't sex them when juveniles. You would want to start with juveniles. If the tank is like a meter long or less, don't even try.
Yep 29g 110L would be way small even for dubs.
Lots of other choices suitable for that sort of tank.
Why no save yourself a lot of heartach and go for cichlids that suit your tank?
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