1) So the best is that I remove the CGA card from the IBM 5155, that I modify the parameters of the switch in VGA and that I put my new card OAK OTI 037C.
Yes - see below.
As an experiment, I have just now tried CGA+VGA. Results follow, and it shows at least one problem.
I no longer have an IBM 5155, but I have an early IBM 5160. The early IBM 5160 has the same motherboard, and same BIOS revision, as the IBM 5155. My early IBM 5160 has an IBM CGA card fitted, just like the IBM 5155. I also have two different VGA cards that are
compatible with the IBM 5155/5160.
I added the first compatible VGA card and connected a VGA display to it (i.e. computer contains both an IBM CGA card and a 5155/5160 compatible VGA card).
At power on, the VGA display is the 'active' display (screen changes go to it). It shows the RAM count-up, etc. The CGA display only shows a flashing cursor.
If I then enter "mode co80", the CGA display becomes the 'active' display - a "C:>" prompt appears on it, and screen changes go to it. No further screen changes are seen on the VGA display.
But a problem I saw is that the VGA display showed programs in monochrome (no colour). There may be other problems as well.
I observed that I could swap from the CGA display to the VGA display by entering "mode mono".
Altering the motherboard's video switches from the CGA setting to the EGA/VGA setting changed nothing.
Swapping out the first compatible VGA card for the second compatible VGA card changed nothing.
So, the above indicates that you will need to remove the IBM CGA card before adding the compatible VGA card, and alter the motherboard's video switches from the CGA setting to the EGA/VGA setting.