The poses look better now, but it looks like you added a drop shadow to Classic Sonic, not an outline. To add a simple outline to any layer in Photoshop, select the layer you want to do it to (in this case, the layer with Classic), then go up to the tabs on top and select "Layer". From the drop down menu, select "Layer Styles", then select "Stroke". Choose the color black and then up the width of the stroke line to roughly match that of Modern Sonic. I'd say it's between 2-5.
But in my opinion, I think the art styles look fine together.
I feel like you need to do something with the sonics, them mixed with the background makes it look way too two dimensional. I love how the poses match but if you need to give that up for better sonics then I would do it.