I want to make a quick report in case anyone in a similar situation finds this thread. Really doubt that will happen, but who knows. :) I have managed to important my old save into SSE and made a new one. Then I added all of the mods I wanted. My mod list is pretty small, only 36 of them, but that was the point. I only wanted to add the most essential mods. I also wanted to minimize the chance of stuff breaking. Plus, my PC is a bit outdated so I didn't want to overload my game performance wise. I've clocked in almost 80 hours into the game. And I had almost no issues with the game. The only mod that I couldn't get to work was Convenient Horses. Which is a shame because that's one of the mods I wanted to try out the most. That's okay though, as I have found a different mod that solved my problem that I originally wanted to solve with Convenient Horses. It has nothing to do with the horses themselves, but with loot management. I thought that using Convenient Horses would allow me to adventure for longer and carry more loot. The mod I ended up using instead is called "Rich Dremora Merchant", which increases the amount of gold that the Dremora Merchant you can summon carries. In those 80 hours I completed a lot of quests. I played through Dawnguard, completed most of the side quests of Dragoborn, a lot of vanilla side quests and I finally finished the main quest. Playing through the game, I found no issues or odd behavior. Apart from Convenient Horses, all my mods were playing nice together and were able to bring me a better and richer Skyrim experience. Which is what I was after. So in conclusion, I think it's safe to mod a vanilla Oldrim save. Just be careful when adding mods, create backups, and don't add too many content heavy mods just to be on the safe side.