A lot of things were already said about Illya’s patronymic. He has canon patronymic – Illya Nickovitch. But we don’t have much people here with such patronymic. It is so rare that even google has very few results on it and most of these results are about the surname NickOvich. So in Russian fandom we call him Illya Nikolayevich (from Nikolay – ordinary Russian name). But still Nickovitch is canonic ))
Next, I think it will be interesting for you to know more about the word “tovarisch”, as it is used very often in fanfics. Of cause Soviet people called each other that way, but only in some particular cases. Friends didn’t call each other tovarisch. Tovarisch was used to call unknown people, to get attention of the audience (Tovarischi, vnimanie! Attention, please), in some corporate context (tovarisch Ivanov will show you around). For example, Waverly could call Illya “tovarisch Kuryakin” – that’s ok for superior to call his subordinate tovarisch+surname. But if Napoleon calls Illya tovarisch Kuryakin, it has some grade of irony. So, if you want to make the situation, where Napoleon is teasing Illya to underline his political views or political views of the country he is from, he could call Illya tovarisch. But if he wants to call Illya out for lunch for example, he should not say “Let’s go for lunch, tovarisch”. It’s the same as “Let’s go for lunch, mister”. Tovarisch doesn’t mean friend or fellow or mate in this case. If there isn’t enough examples here, I’ll try to put several more later.
In some fanfics there are situations, where Illya is using Russian swear words. In our opinion Illya hardly would do that, but if he really do he would use them very inventively and accurately. He’s very cultured person in canon. And in Soviet Union swear words were not accepted, especially in the families of Russian intelligentzia, and also in ordinary families, in presence of women and children. And the most inappropriate way to make Illya say swear words is to make the direct translation of those words from English to Russian. For example: Illya’s saying “Shit, I’m late”, but the word “shit” is said in Russian. “Govno, I’m late”. We don’t speak like this. It’s very unnatural phrase. So this is very hard even for Russian authors to write such parts where Illya’s using swear words, and that’s why I even don’t know what to advice. But we’re always here for you to help :)