That all sounds good, it’s great having a family member who knows a little of fishkeeping helping out.
And hearing of your tank being cycled is all good to hear, brilliant. So that’s ruled out for certain. One less thing to rule out.
No one here, especially not me anyway can claim to be an “expert” in fishkeeping, anyone who says they are experts casts serious doubts about that imho!
Any, I digress, it’s not too fast to be fin rot or TB, though I think it’s much more likely to be fin rot rather than TB, as the symptoms differs vastly.
Fin Rot - signs of symptoms are fraying fin edges, base of fins appear inflamed, edges of fin may have white or black edges or fins may also have white dots.
(if these white dots are also seen on the body it is possibly a symptom of ich)
Early signs of TB being fish being really thin, twisted spine deformation and missing scales and tissues and open wounds and blisters especially in top half of body. Abdominal distension is another clasign, along with, missing scales and open wounds and blisters on the top half of body to name but a few of the symptoms, this does not sound like the case here to be entirely honest.
And also not knowing the water hardness and water parameters makes it difficult to discount fin rot or water softness for guppies for certain.
So that’s the first few things we need you to provide.
Water hardness (usually supplies in different units such as mg/l, dH and degrees Clark) from your water suppliers.
Ammonia, nitrite and nitrate was well as pH may prove to be useful.
And then we can take things from there.