Compatibility of Syno Multipunctatus and Severums. Ph of tank is 7.2. Doable?

Hi. I have a group of 8 , 5-6 inch, Syno Multies ina. 90 gallon tank. They are in with 3 inch fish. I would like to grow out some fish in that tank. What size would you recommend them to be to not get eaten? Same question for my 4, 3 inch Lucippinis. Need to move fish around and have some 1 inch fry...