Which Country are you from?
55 years is stretching it, not sure even I have seen that large a gap, but it is common in such relationships for the USC to be significantly older and they do seem to be more lenient with LGBT relationships.
F1 is the student visa but it is non immigrant
a parent petition will take a couple of years so the youngest one will be F2a and the oldest most likely F2b
I do not know if she can adopt them or not but irrelevant for the topic in hand