[Plinkploop]

Short answer- no. I just wouldn't trust it... Things could be fine for months and then all the sudden there's a complete massacre. I wouldn't keep dempseys with anything long finned, especially in a tank that small.

 

[Plinkploop]

TY. I thought the Electric Blue was less aggressive?
Will it be ok in 40 g breeder?

"Less aggressive" referring to any larger cichlid is speculative at best, marketing ploy at worst. He (she) should be okay in the 40 for a while. I, honestly kept mine in 60 gallons or above when even thinking about keeping more than 1 large cichlid in a tank.
Keep an eye on the parameters with keeping a Dempsey in a 40 gallon, they tend to be messy. As long as you keep up on maintenance and over size your filtration you should be able to get away with it.