Of course you can swap a Shimano for a Suntour, and a six for a seven, if you feel it's needed. Since you haven't ridden the bike yet, I presume, you may find the shifting is just fine. I believe it's set up with triggers and a flatbar, so you have some work cut out for you before you can actually tell how it works with barcons.
Both freewheels are suitable for a 126 spacing. Depending on the wheel & frame, you may have to add a spacer or two to the axle so that the freewheel clears the frame, you may have to redish the wheel as well. Trying is the only way to know for sure.