If I just insert the card in the PC and delete some photos inside DCIM folder, or add some, can this damage my card or camera? Should I use some special software for this?
There should be no problem at all if you copy or delete files from the card, but each camera uses particular ways of managing data in the cards, so writing, adding or editing files with a computer or other devices may interfere with some functions of the camera.
Using Windows Explorer or another file management utility gives you the best flexibility and often faster operation than the bundled software of certain cameras. This flexibility may allow to perform operations that the camera is not prepared for. Normally, none of such operations may cause damage to either the card or the camera, they may in worst cases prevent the camera from performing certain operations or have unexpected results and cause data loss.
Reading a card will not cause any trouble, as long as the computer, the card, the reader and any cables used are in good condition (there are no virus, damaged physical conductors, etc.)
Deleting files is also safe, but you must remember that Windows OS does not "send to recycle bin" any file that is in a memory stick, usb drive or memory card, so deletion is definitive. That's why the advice here is to be very careful to delete only files that you have already made a backup or to be very precise with your selection and deletion procedure.
Many cameras won't display or read pictures that have been edited by third party software (even pictures rotated with windows picture viewer won't be displayable for some cameras).
Some cameras have "post production" options in-camera, but some cannot apply such filters to pictures that have been taken with another make (or even model) of camera. Conversely, such in-camera edited pictures, may not be viewable/editable in another camera. Some cameras won't even let you re-edit a file or apply the same "filter" again to the edited picture, but most of them will preserve the original file and save an edited copy with a different name.
Another concern is file names, if by any chance you place a file in the same folder the camera uses for it's own pictures an this file has a name the camera will use later, results may be unpredictable. In the best case, the camera will use the next available filename, but you may (depending on the camera programming) loose the file you recorded first, or loose the current shot.
Depending on the number of files with conflicting names and file name generation logic of your camera you could cause some performance issues, for example the camera taking too long to record a new shot.
However, most cameras use a two or three level folder system, typically in the root of the card, they place a folder named like the maker of the camera, inside that, another folder named "DCIM" or something similar, and even inside of that one some other folder with a name like XXX100, XXX101, etc...(Or some other combination of names)
Usually you can do whatever you like outside such folders, that is, create another folder and copy any file that fits in the available space. Normally the camera won't even notice such files, so they may be pictures, music, documents... The downside is that the camera won't be able to delete, move or copy those files, so if you run out of space while taking pictures, you'll be forced to delete pictures from the camera's folder or downloading them to a computer (Either case you have to shop shooting).
If you really need to edit pictures in the same card and folder, or need to write files in the same directory of the card used by your camera, it is advisable to make an appropriate backup and perform some test to be sure to know the camera's specific behavior, so you know what to expect afterwards.
(Based on first hand experience in use of several models and makes of cameras including Canon, Casio, Nikon, Olympus, Panasonic, Samsung and Sony. In most cases, user manual did not mention anything regarding this kind of issue.)
No. it will not damage your card or camera.
The worst case scenario is that the camera would stop reading the card and the solution is to copy anything important on the card to your computer and format the card in camera.
I've been doing this for years with lots of different cameras and never had any problem.
No it shouldn't be a problem.
I copy/cut all of my photos this way, the only time when you might have a problem is when you say rotate the photo and leave it on the card, then the camera might not be able to show the image in preview.