A convict or JD would be okay. I see some say do not another fish. I say if you are going to add then do it now. Because once the other really establish their areas of the tank you may not be able to add another fish.
A male convict may help keep the Salvini in check for a while and bring some balance but the Salvini will 100% go nuts some day. That firemouth needs some backup hehe. The GT is the unknown factor.