Not all treeless saddles need a special pad - it depends on the construction of the saddle. Some of them do need a special pad which basically raises the saddle up a bit to clear the spine. Hard to fit horses sometimes need customized padding (many treeless saddle pads have pockets so you can add extra padding as needed), which will take some experimenting to figure out.
Treeless saddles do need to be fitted. A bad fit can cause back soreness, just as with a treed saddle.
The Stallion Riders saddle is a knockoff of the Stateline tack saddle, so it may be okay in the short term. Here's the Stateline Tack saddle: Down Under Kimberley Soft Saddle - Statelinetack.com