The slacker dock only supports USB-IN, it doesnt support anything else on the S5/S7
Things missing from the slacker dock (that's available on the 2nd gen official dock):
- USB-in (ie OTG)
- USB-high speed charing
- Dock sense (where the device automatically goes into clock mode)
The slacker dock has a headphone out, but the S5/S7 dont support it either (it may not have the pins for that, if it does it's different anyway). The slacker dock also has pads for USB-in, but that model was never officially made available(?).
Also I would STRONGLY recommend against a slacker dock for the S7 (if you can even find one), the S5 is a lot lighter then the S7.
The S7 barely fits on it, but it has absolutely no support besides the port itself.
The S5 already places all it's weight on the dock port, the S7 is asking to be broken by any accidentical impact.
If you are willing (or can find it at a reasonable price), I would strongly recommend the official 2nd gen dock for either the S5/S7.
It includes 2 backplates to support either model.
Also, the S5 dock in the previous link likely wont fit the S7, that's the reason the official dock comes with 2 backplates.
Your only real choice for the S7 is the official 2nd gen dock.