Nope, she won't spin dash because of what you mentioned, it would look just plain weird. :P Sally's stats will be somewhat similar to Donnyku's Amy, but without the slow fall. (Which would also make little sense if I left it in there.) Including the fact that Sally won't have any ability or spin frames, she'll need less than 90 sprites to be a completely functional character.
Sorry I have no sprites to share with you guys right now, I'm trying to whip up a quick playable Alpha. We need to see how well she plays before the final release, I want her to handle as uniquely as possible from other slow characters. If any of you have some S_SKIN tweaks in mind, post it up. :P