M.R.
While I can't speak from a foster/adopt background (which incidentally is what we will be starting on next year), I can speak as the biological mom to 3, with a fourth on the way.
They ALL regress. It doesn't matter what their circumstances are or what background they are from.
But, the good out weighs the bad. As long as you are able to reassure them of continued love and affection, and show them how to love and affect others, then you will make a wonderful mom to baby four.