How should I attach heavy floating shelves to hollow plasterboard walls?

I have the following shelves, intended to be floating shelves, that we want to attach to our kitchen walls. The walls are hollow plasterboard, with no studs where we want to put these shelves.

They came with fixings like these, which are intended for solid walls or studs, 2 each for 2 pre-drilled holes in each shelf.

The measurements are as follows:

• shorter shelf: 5.5kg, 75x22x6.5cm
• longer shelf: 7.5kg, 105x22x6.5cm

We won’t be putting anything extremely heavy on the shelves, but they will probably need to hold quite a bit more weight than just themselves, just in case (maybe double their own weight?).

I’m guessing I’ll need to use a different fixing for these, one that is designed for hollow walls.

My questions are:

1. Is it reasonable to expect these to work as floating shelves or would it be more secure to use brackets (not totally opposed to this, but would prefer another option if possible)?

2. If yes, what kind of fixings should I look for to securely attach these shelves to the wall?

3. Would it help to drill any extra holes in each shelf so that they’re attached at more than 2 points?