I can tell from owning Frontosa that there is almost always a reason they die that can be figured out.
At 1", I would personally not have mixed them with the cichlids that are currently in that tank. African Cichlids are very zippy, and aggressive. Frontosa, although an African Cichlid are very calm and docile cichlids, and they can stress very easily. It is VERY likely that they are stressing out, or being bullied by the other fish.
They are 1", and your other fish seem bigger, so internal injuries from the bullying is very possible. Which would cause the swimming issue.
Although you say they do not attack them because you watch doesn't mean it doesn't happen, as we all can't just sit in front of our tank all day, so there may be something you are not seeing.
I would recommend moving the 2 you have if possible.