Worked perfectly! Now it works in fullscreen. One more thing: I see, is that, for example, the Cosmo Canyon backgrounds and the Cosmo Area fighting backgrounds aren't replaced. It also happens at the beginning of the game. The world textures seems to be fine. In case it helps, here are my options, I tried tweaking a little, but counld'nt solve the problem:
[OVERRIDE]
Disable_The_Reunion = n
#When using Windowed mode, the recommended resolution is 1280 * 960.
#When using Full Screen mode, the resolution will be changed to your desktop resolution.
[OPENGL]
full_screen = y
window_x = 0
window_y = 0
window_width = 1920
window_height = 1080
preserve_aspect = y
vsync = y
texture_cache_size = 2048
compress_textures = n
save_textures = n
use_pbo = y
use_mipmaps = y
fancy_transparency = y
post_file = psx.post
#on-screen logging
show_reunion_popup = y
show_reunion_info = y
show_fps = n
show_stats = n
popup_error = n
popup_info = n
popup_ff7 = n
trace_personal = y
trace_open = y
trace_textures = n
trace_files = n
trace_other = n
opengl_debug = n
[CUSTOM]
Mod_ID = NONE
[AUDIO REPLACEMENT]
Mute = n
Music_Vol =75
SFX_Vol =100
Mon

utput = n
Centre_Battle_SFX = y
Field_SFX_In_Battle = n
Music_Balance_Functions = n
Load_SFX_To_Memory = y
Audio_Log_Level = 4
# Weapon mod: 0 = Off / 1 = Minigame updates / 2 = Further field updates / 3 = full mod [will do nothing at this time].
# 60fps Battles mod: 0 = Off / 1 = On (no interpolation) / 2 = On (interpolated model animations).
[MAIN]
New_Translation = y
Model_Overhaul = y
Weapon = 0
_60fps_Battles = 1
[TWEAKS]
Centred_Field = y
New_Gauge_Colours = n
Closed_Mouth_Models = y
QPC = y
Game_Time_Is_Realtime = n
Transparent_Dialogue = n
[FIXES]
Battle_Arena_HPMP = n
Battle_Key_Press = n
#For the poor souls with epilepsy
[DISABLE SCREEN FLASH]
Critical_Hit = n
Boss_Death = n
Summon = n
Transition_Effects = n
Disable_All = n
[OTHER]
#For Windows Vista and Windows 7. Use this whenever possible to improve frame rate.
Disable_DWM_AERO = n
Also, the menu doesn't look overhauled, both the battle menu and the game menu.