If I were to replace 2 new neon tetras (which died), would the other 6 in the tank accept them into the shoal considering they would probably be about a third of the size. What about a similar situation with a bristlenosed catfish (there is one left I want another) although I might be able to get a similar sized one of these . Advice much appreciated.
Feed your original fish before adding the new ones, with the bristlenoses turn out the lights for a time, until they've settled, they will scrap for a while
I introduced a smaller female platy into my established tank (my existing females are HUGE...the new one was about 1/2 the overall size) and believe it or not, she actually bullies them more (small fish syndrome )
same as my tetras....my neones have accepted new neons I have added....they look at them as if to say "are you new?" the new ones look back as if to say "yeah, you got a problem with that" then they forget and all is good!!
. Advice much appreciated. 
Feed your original fish before adding the new ones, 
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