If a single engined propeller plane has wheels under it, it's an SEP. The fact that the wheels fold into floats does not take away from that, nor invoke the need for an additional rating - unless you want to land on water.
That said, using an amphibian version of a Cessna is a less good (and probably more expensive way) to gain experience on type. The landing visual cues will be very different. If the jump outfit can put the dual controls and right seat back in, they'd probably love the revenue from some type training...