Think it is the bacterial starter that gives the false impression your tank is cycled.I had a beneficial bacteria starter that works.
My two cents about the stocking and such.
28C is too warm (by far) for all mentioned fish old and new).
The male DG will be constantly in matingmood (induced by temperature) with issues cause of that. Agression, wanting to build a bubblenest, chasing the female raround and maybe even killing it. Also there are no floaters and no plants as hidingplaces for the female after spawning.
The footprint ot this tank is rather small for Corys as well is the numbre insufficiant in my opinion.
I'd seriously would rethink about stocking.
The mentioned small barbs would be great (lower temp)