Hi there
I personally don't think starting solids before 6 months is a good idea - and research suggests that it will make no difference to night wakings.
I think that there will be times that your 5.5 mo may still need 2 feeds during the night. My DS feed up to 2 times per night until he was 9 months old, and still wakes most nights for a feed (and he is almost 1). I suggest that you try hard to make sure that your DD falls asleep independently at the beginning of the night, and that you respond quickly to her when she wakes. If you are unable to settle her reasonably quickly, I would suggest you offer her a feed.
Because you are bottle feeding, you can use the slow diluting method detailed in the post at the top of the board.
Good luck
Jo