Can I add more fish to this tank?

I have a 54 litre (12 UK gallons), pH 7, 24 degrees Celcius tropical aquarium which currently holds 5 Cardinal Tetra, 4 Penguin Tetra, a Fighting Fish and a Bristlenose Pleco. Would it be possible to add any more fish, in particular Tetras? I'm open to other suggestions too, though.