Dortmund has several of the same strikers as Guirassy. yeah, the performance of Sebastian Haller and Niclas Fullkrug couldn't be relied on by Edin Terzic at that time. As for Donyell Malen, he is actually better at being placed in the winger position than the other two strikers. In fact, last season Malen only started 21 times.
Niclas Fullkrug is a different player striker type as Sebastian Haller and Donyell Malen are.
I guess that maybe Sebastian Haller or Donyell Malen will be leave Dortmund but on the other side you need players for the whole season.
Its possible that that all players will be stay and that looking for a tactical playing with different types of players.
But Guirassy is an improvement for Dortmund.
To be honest, when I first saw the report that Borussia Dortmund were seriously negotiating with Stuttgart in other to sign Serhou Guirassy, I was very surprised not because the Guinea international doesn't worth the chase from Dortmund but because I thought Borussia Dortmund already have world class strikers in Niklas Fullkrug Sebastian Haller. Donyell Malen like you rightly said mostly operates from the wing but for Fullkrug and Haller, they're all top 9 strikers.
Looking at how impressive Serhou Guirassy was last season, I think it was one of the major reasons Borussia Dortmund went for him which I think is a good move as they try to strengthen the team however, I don't see the club retain Sebastian Haller and Niklas Fullkrug alongside their new striker.