Is this modpack compatible with Realistic Fusion Power mod? And is there any possibility that you will add it in this modpack? I really would like to get complex fusion reactors here

The mod itself looks lit and the compatibilities with SE and K2 seem to be given. So without trying it out, it should work with each other in principle. It could be that there might be problems with HMM-Nuclear (or with any random mods), but you'd just have to try that out. Feel free to give it a try and let me know how it goes and if there are any difficulties (error messages, duplicate items, settings to be set reasonably etc).

I'll test it out myself as soon as I can. But since it's more end-game material and I'm nowhere near that far in my world with the modpack, balancing would be a bit harder for me. ^^'

So, there is incompatibility with current version of SE, it is because SE deleted "t3-tech-cards" and replaced it with "science-pack-creation-2". However there is a manual solution for that. It can be fixed by unpacking the mod zip and replacing these two words in the following files: "compatibility-patches\Krastorio2\data.lua" and "compatibility-patches\space-exploration\data-final-fixes.lua"(Solution by Chatastroph). But I am not sure if you are able to fix it within your modpack by settings of something.
https://sun9-76.userapi.com/impg/5aqYO-jzk-psmbxkW9k0oRV8ECwZD6y7vCewfA/quSm5zF-ga4.jpg?size=936x380&quality=96&sign=3fed6b8d721b21ae888bee25626195bc&type=album

Despite that this mod is highly compatible with K2 and SE, and even by default replaces standart fusion/antimatter power from K2(can by disabled via settings)

I got the same result when I checked it myself earlier. Okay, so basically I won't want to add it with the incompatibility for now, since the modpack is supposed to be a click&play experience. If you want to tinker around in the files first, it's because you really want this mod and then you'll probably just install it yourself and you won't need in the modpack in the first place.
But that doesn't mean it will stay that way in the future. I refer to the thread that has a similar problem with 248k: https://mods.factorio.com/mod/heavily-modded-modpack/discussion/63fd5f30a50ef63fcff35fc5

I hope the problem will be fixed soon or there will be a workaround in the modportal. Until then, I also think it's a shame that it's on hold. :'(