the tetras are fine they are really happy and loving be the only tetras and the red tailed black shark is possibly 2" also before i got all those fish it was a coldwater tank and it had a 1 foot sturgeon a minnow and the pleco sort of thing and they were all fine so i dont think i need to get rid of the red tailed black shark.
The red tail shark will get quite large and will eventually start to bully the other fish.
My friend started out with a 2 inch red tail last year. This year it is at 6 inches and it eats his guppies and other fish about the same size as guppies.
The pearls can also get to about 4 inches so I wouldn't add anything more if it were me.
the tank is overstocked and many of the species are not compatable...serpae tetras can be very aggressive and will certainly nip the fins of the other fish..the buenos aires will also nip fins.the pleco is most likely a common and will get too big for the tank....the redtail will also get too big and will go after smaller fish as well as the angels and gouramies..
but after all , it is your tank , your fish , your time and your money...it is your choice to put what you want where you want...