Only certain cukes like Sea apples nuke a tank if they die, not all have that issue. I've had one in my 40 breeder for about a year now and it chills in caves during the day but at night he comes out and sifts the sand bed. I'd make sure the tanks pretty mature before adding one but I don't see an issue with it in a 29. Mines about 5" long and pretty fat but I doubt it will get any bigger. It's a Caribbean species and kinda looks like a turd but they're cool animals.
20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"