would like to know the truth about it:
some people say that when tapets are removed in order to be cleaned, you don't have tu put them in exactly the same order cause they are hydraulics, other people say that it is important to install them in the same orde.

so?

Everything I have read has been quite clear: tappets must be replaced in their original locations. Same goes for valves.

When I stripped and rebuilt my T16's cylinder head, I made sure that all valvetrain parts went back in their original places. That means springs, valves, tappets, seating washers, collets, top and bottom spring washers.

If you're taking out the valves too, then I'd recommend fitting new valve-stem oil seals.