When we first started seeing booties around, there was a ton of debate on the forum about that exact issue. I think ultimately it came down to "some gap is okay, just not to much...at least for most." There are a small contingent of us who don't mind more of a gap with the right bootie type (I'm in that group), but the majority felt that the gap shouldn't be more than say 1/2 inch.
Alaskagirl (Una) has the same issue. She has very slender ankles. You could check out some of her posts.