Typing skills are fine to me
As a general rule 1 fish approx 1- 1 1/2 inch long needs 1 gallon of water so here in U.K thats 4.5L.
So you could house maybe 4/5 adults at 2 - 2 1/2 inch long again depending on adult size of fish.
People forget that when the fish are young they are fairly small & LFS feeds are minimal so when you get home & start feeding daily with better foods,After a few weeks/months they have probably doubled in size
The Bolivian Tetra can grow to 2 inches so again this reduced your tank space before you added the new fishes
Plus they may need more than 2 to feel safe
The Emperor Tetra is 3 Inch when adult sized, So a guess is there around half of that now.
I do think that the lack of space combined with New tank syndrome has killed the Emperor
Ive done it myself in the past, Hope you get sorted
Im not judging you by the way as you did the acclimatising right & you used established media,Also you have fish that are a few years old. So you must be doing something right