you can use a tear away- or a wash away- or a fusable that stays in- or a fusable interfacing---what ever you want- if you choose a stay in fusable make sure to use one of the 'light' products---like steam a seam lite or heat n bond lite- the other ones (heavy) are not sewable...and cause your project to be very stiff-they do not soften up when washed.
tear aways & wash aways are ok too- but you do have to deal with the removal.
personally i pretty much never satin stitch my appliques- i generally use a small zig-zag or a blanket stitch- i don't like that heavy thick line of stitching caused by satin stitching-but that's just me- you can do it any way you want/like...just be sure to choose a sew-able interfacing.