It'll be an ELC since it's an age required thing in the CBA. As I understand it, a European (trained) player would have to be over 28 years old in order to not sign an ELC. Couple examples off the top of my head are Vadim Shipachyov (2 years x 4.5 mil AAV when he was 30) and Evgeni Medvedev (1 year x 3 mil when he was 33).
When Calgary signed Roman Cervenka at 26, he still had to sign a 1 year ELC. Nikishin at 23 would have no option other than to sign an ELC. Contract length is also defined by the CBA, so that part wouldn't be negotiable; 18-21 year olds get 3 year ELCs, 22-23 year olds get 2 year ELCs, 24+ get 1 year ELCs.
The main question with Nikishin is whether his first year would start in 2024-25 which would potentially have him be an option for the 2025 playoffs a la Nikita Gusev joining Vegas in 2018 (albeit he never suited up). Even if he weren't to appear in any games, Year 1 of the ELC would be burned which might be the cost of luring him over; Minnesota burned Year 1 of Kirill Kaprizov's ELC without him playing.
This is where the "Russian Factor" can come in. The CBA rules means he will sign an ELC deal when they are 22-23 or whatever, and likely have to leave some money on the table to do so compared to what he could likely get in KHL salary. Now players are willing to do this in order to chase the upside of a big NHL deal, so you basically never see guys that aren't willing to take a go at it. Since it's an ELC, he is also going to be waiver exempt.
When these guys come to camp, the coach is (not entirely wrongfully) just looking at them as another young guy that hasn't seen NHL action yet and doesn't care what they did in the KHL. So they are generally put into depth roles accordingly, like you would with a random player that comes from the AHL. Since they are on an ELC, they are also waiver-exempt, so when roster shuffling needs to occur, the roster politics of it will put often put on the shortlist to be sent down to the AHL if they haven't done anything yet because of their contract/exempt status. This is all made especially more exponential if the Coaches have a prejudice that presumes all Russians are entitled/don't work hard/greedy/selfish until proven otherwise.
So they go through a season of that, and then since they haven't done anything particularly notable to date in the NHL they aren't going to get a big contract offer, and will likely be offered a deal close to league minimum without assurances of a large role for the upcoming season. At that point, the cost-benefit analyis may weigh more in terms of "whatever, I'll just go back home, get a big role and make more money", so they go back to Russia, Garl does his celebratory dance about "another Russian flop" and that's the end of it.
Now, this doesn't mean a very talented player isn't going to come in, impress everyone right away and earn a big role because it'd be coaching malpractice to not give them a big role and then they get their big contract and everything works out. This happens all the time. So the "Russian Factor" is less of a factor for really talented players that are clearly amongst the top 100 players in the world, for instance. It has much more of an impact for would be depth players.