karnaugh maps simplification
That is allowed. If it was not allowed you would have no way to deal with rows or columns of odd entries since your entries must always be grouped with even dimensions (except for single rows or columns).

The formula looks good to me. 

Google K-maps if you're ever unsure since there are now a LOT of really clear examples compared to when I was in school which was not even that long ago. Either that or it was there but I was too stupid to realize I could look up material in places other than a textbook for help.