Does getting bigger fish help them survive more?

As posted in my previous question, I have had my tank for almost 3 years and everytime I add in new fish they die. I have a semi-aggressive/ community tank with 1 opaline guarami, 2 kissers, 2 neon tetras, a glass catfish & 2 striped raphael catfish. I've tried adding other tetras, platys, mollys, sharks & more but they all bearly survive the night. Should I try bigger fish? Any other suggestions?