I want to start modding the game, but it seems installing the game through mod galaxy is not compatible with modding, so can i install them anyway, or what dowloader is better to use when we want to mmod the game?

galaxy won't even install a functioning copy for some people. if you want to add the unpredictability of mods on top of that then that's your choice. i'd personally make sure the game is stable at every stage. i would not use galaxy, and install mods one by one until i can verify the quality of the game and mods.

I play it on Galaxy and had 32 mods installed via Nexus mods.

I just manually downloaded the mods, then copied each one's folder manually into the game folder on my PC, then ran it throgh the Witcher 3 Script Merger (a mod from Nexus, but NOT their on site mod merger as that doesn't work for many mods; it's a download that a member of Nexu made, but I can't remember where on Nexus I found it. You should be able to search for it on Nexus to fnd it however.)

Of the 32 mods, 28 of them turned out usable together, the rest conflicted and were unable to be merged so I removed those.

I'm now 250+ hours (33%) into the game and have had no trouble playing it on Galaxy with the mods.

ETA:

Not sure if it makes a differance, but the mods I'm using are all "cosmetic" mods that don't change the gameplay. Like I'm using the "White Wolf" mod to make Gerault look lore friendly and pale like in Witcher 1 and I'm using the "chrome mod" that makes his sword gleam like chrome, I'm using a "Frea's horse" mod that turnes Roach into a glowing purple unicorn; one that makes Gerault's eyes lore friendly yellow and cat like instead of the hazel colour in the game. That sort of thing.

So, I know those types of "skin changing" mods work. But I don't know about the mods that actually change game play mechanics as I've not tried those types of mods.

thanks! i'll try to find
how did you install it manually, my game files are separated in 12 files?
did nexus found the game while scanning? the game works correctly for me, but i don't know where is to manually install the files in my game!
i don't want to modd the game to much, i want to keep it fast and stable also ^^

you mean the game? galaxy allows you to download a backup of the game. otherwise known as the game installers. you can choose to do this via the browser also.