More often than not, examiners/judges will be quite lenient. They know you're shaking and very nervous, so they grade according to that. I played Mozart K.283 in a festival last year and made mistakes in all the critical places, yet still received the highest ranking.
About sight reading: Practice in your spare time (when you're too tired to do pieces AND scales
) and be as confident as you can. If you don't think you can be confident, fake the confidence. Always look at the score (especially the key and time signature!) before you touch the keys, and imagine what the first line or so should sound like. Identify chords/other-fun-music-theory-jargon if you can, and fake it according to those-i.e. If you see a C arpeggio in the bass that keeps going but changes in the order of the notes, try to play C/E/G in places that sound right rather than not playing the bass. Most importantly, take a reasonable, cautious tempo and stick to it.
About the music history part: It sounds like fun! (This is coming from a music history freak.)
Baroque: In my mind, baroque has lots of little trills/mordents/ornaments that frequently occur where it goes 'ti-do' or '7-1'. 2/3/4 voices are frequently used, but composers (Bach, in particular) have a way of making it sound very simple and clear, like Lillie said.
Classical: Whatever isn't baroque, romantic, or modern. I think it's very hard to define-but if you hear Mozart (broken thirds and childish simplicity are his trademarks, I think), it's classical.
Romantic: In my mind, romantic is full of large chords, distinctly-un-baroque trills and ornaments, black keys, and emotion. If you see the examiner's hands spread to ridiculously large intervals/chords, it might be Rachmaninoff or Liszt. Chopin's etudes sometimes go all over the keyboard, though that's not definite.
Modern/20th Century: Ughh...anything ragtime is obviously 20th century, but other than that, I don't know. Say Shostakovich or Bartok (is he considered modern?) or Prokofiev or someone like that..
I've never taken the ABRSM exams, but I heard that only grades 5 and 8 were worth taking. I would try grade 7-Wouldn't doing grade 6 again be awfully boring? Besides-you probably passed. Don't worry about it, and have fun with piano!