Can I stack several devices on top of each other on the shelf?

You can sometimes do so, but I would not turn the shelf into a precarious cairn of routers merely because the combined weight is under 15kg.

Each device still needs a stable footing, cable access and whatever ventilation its manufacturer requires.

Two items designed to stack neatly may be perfectly reasonable; a procession of mismatched boxes leaning on one another is a different proposition.