Slaf is playing like he belongs on the first line. I agree, keep him there. In fact, with Caufield struggling to score, you can view Slaf (playing as he is at present) as significant "value added" to that line.
Ylonen - he may be no more than a "tweener," but like you, I have a good feeling about him. He seems like a smart cookie who may grow into a role as a shifty 3rd-line/2nd line FW.
I don't want to be premature, but it does seem that, for all my hand-wringing, this management group is actually developing its young talent properly. The young D are coming along. Slaf has finally started to show why he was drafted where he was. Like Dach, Newhook was growing into his role before (sigh) suffering disastrous injury. Even Primeau, as you say, is surpassing expectations.