I Know that its more popular to retopo after sculpting, but should I also try to retopo a high poly model that was created without sculpting, or I should try to use some addon to create lower LODs for the model, or something else??
THX 
Welcome! 
It really depends on a model. If it’s all quads you might be able to use decimate modifier with un-subdivide setting and rise the iterations until it’s low enough without losing detail or having glitches. Or even use retopo modifier with right settings. But if you cannot get a good topology this way then you can ofc retopologise it manually. 
Is it for renders or for animation? For renders, it is not needed; for animation, it will make things easier.
Asking for models for Mobile games.
thanks!
Thanks!
Ill try it out.
What do you think about LOD add-ons?
Because the “decimate” modifier have a history of giving me gibberish results, maybe its because the model wasn’t that high poly(like a game asset) but if that’s the case, then from what I understood from you is that I should just retopo manually or straight cut of vertices without loosing the shape.
Don’t LOD manually if you’re making stuff for mobile games, let the game engine handle it