Like Harold, I see people in flip-flops in my .gov office as business casual. I'm thumbs up with the boots though. I did khakis, polo or dress shirt, and boots at a previous job where the dress code was a bit more formal, and no problemo there. In fact because I had to go out to field sites a lot where steel toes were required, I often wore the Red Wing "semi-dressy" cowboy boots with the steel toes. Sounds like you may have the same potential issue given your job.