Some species of synodontis (in my experience) can be very territorial.
They can be with each other especially if you only have 2.A lot of them are better alone or in groups of 3 or more.
I have also known them turn on cichlids if they both decide they want the same hiding places.
They can and do work in cichlid tanks, but there is a chance it may not.Still worth trying if this is the fish you want.
Pimelodus would just be a safer option.
Hoplo catfish would be my first choice. They worked really well with my rivulatus.