Tetras and other social fish IME usually do best in larger groups. 3 is not a very big group. You can try 4 fish, but watch out for nipping. Shoaling species may nip more in smaller numbers.
And C julii may be a bit crowded in a 16g. Assuming you are even getting true julii and not C trilieatus or another lookalike. And Yes, Cories like to be in groups of 6 or more just like all shoaling/schooling fish.
84f is high temp. You should lower it to 76f before adding Neons. Less oxygen in hotter water. Don't buy all the fish at once. Small tanks get out of balance faster. Keep testing after adding fish also.
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