I've got a Fluval Roma 125, which is very similar to the Rio 125, that I am in the process of dividing up for my males.
In my opinion you wouldn't want to divide it into more than 6 sections other wise they become very narrow.
Of course it's possible but it depend on what you want visually.
Dividing it into 6 makes each "compartment" just over 5" wide. I made 5 at 5" and one a bit bigger because I was going to pop the filter (with spray bar) in with one of the PKs. Now, I've decided to run an external filter instead, so that the water is sucked from both sides of the tank rather than all the way through all sections to one end, then returned via a spray bar.