Will skipping the railgun and accelarator spinoffs affect the story? Are both the spinoffs necessary? what is the correct watch order skipping both the spinoffs.
The spinoffs aren't necessary to understand the main Index series, but personally, I enjoyed the Railgun spinoff a lot more than Index itself, so you could be possibly missing out on a lot of great stuff if you skip the spinoffs!
But if you only want to watch Index, that should work, too. The different series are all set in the same continuity, but focus on different characters:
- Index mainly follows Kamijou Touma's point of view. However, later in the series, it starts following Accelerator's point of view too. And later on, Hamazura Shiage's point of view as well.
- Railgun mainly follows Misaka Mikoto's point of view.
- Accelerator mainly follows (surprise, surprise) Accelerator's point of view.
So, if you watch only watch the Index anime, you should be able to more or less follow what's going on.
However, the spin-offs add a lot of world-building and background context to the universe. The various characters from these different spin-offs do often cross over into one another, so having knowledge of what happens in the spin-offs can help enhance your enjoyment of these interactions.
For example, season 1 of Index and season 2 of Railgun both cover the events of the Sisters Arc, but Index only focuses on Touma's point of view of that incident. If you skip Railgun, you'd miss out on Misaka's experiences in the weeks leading up to her meeting up with Touma.
But to recap, this is the release order of the various Toaru verse animes (generally, recommended watching order is release order):
- Index (24 episodes)
- Railgun (24 episodes)
- Index II (24 episodes)
- Index Movie (1 episode)
- Railgun S (24 episodes)
- Index III (26 episodes)
- Accelerator (12 episodes)
- Railgun T (25 episodes)
If you only want to watch Index, you can go Index → Index II → Index Movie → Index III. (Note that Index III adapted about 9 light novels' worth of content into only 26 episodes, so that's why it might feel a bit rushed at times.)
If you only want to watch Railgun, you can go Railgun → Railgun S → Railgun T.
Accelerator happens after Last Order is introduced in the Three Stories Arc in Index season 1, so it's probably best to watch this spin-off after finishing that.
If you want to watch some mix of everything, release order is fine.