I would leave any central americans out of this mixing souths and centrals rarely works out well as we both know now... I dont think you have space for an other oscar really, the Thread Fin Acara is a good shout though. Wharf Aquatics have some in stock too
https://www.wharfaquatics.co.uk/tropical-stock-american_cichlids.htm
For dither fish, they serve 2 purposes, give shy fish confidence and spread out aggression between fish in the tank or distract fish that are fighting in a bid to keep the peace. I think you need to think much bigger than Cardinal Tetras as these will 100% be eaten by the Oscar, you need to be thinking 3-4 inch minimum.
Wills