The look like RBP to me . The latest pics show the spots . I have some very similar in size . I am work or I would post the pics. Mine look very close to yours.
First of all, baby rhoms are indeed possible as right now they are available for sale. The tail on the fish you posted does look like rhom, but the body shape does not... I'd expect a little more rhomboid shape than that. But then again, they could be stuffed from over-eating so it's going to be hard to judge from those pics alone. Also, baby rhoms might have a tendency to pick on each other.
Where did you get them? Aquascape? If aquascape, then they should be a baby serrasalmus of some sort. Likely rhom.
Rhoms grow incredibly slow, and RBP grows incredibly fast. So in a few months you'll for sure know what you've got.