Nogai horesemen are very fief-dependent. Short answer: Nogai troop is only recruitable from one particular recruitment place.
This place is: IZUM fortress, southeast part of the map, northeast of Perekop. NOT -IZMAIL- fortress, that you currently own.
One easy giveaway is that IZUM fortress has a village of NUGAI, right close by
Note: Also, Izum fortress makes more sense than Izmail fortress for Nogai recruitment because most of the vassal Nogai Horde was to the east of the Crimean Khanate.
In my game, I only have 25-30 Nogai ... they are not easily mass recruited. I'd love to find a way how to do so, though. As you said, their stats are what makes them superior, despite their unarmored, frilly look, which is quite misleading. I'd luv to recruit them. In my game, another lord owns Izum Fortress, and he's taking his sweet time upgrading everything there, so this is my rate-limiting step in getting more Nogai (only a max of 5 per recruitment cycle). Is there any way to further increase this number? 5 seems low, 10 would have been better. It would have been better/easier if I had owned it. Any way to give away a fief I don't want and receive a fief I do want?
Historically, the Nogai Horde pastured on the steppes northeast of Perekop, not northwest. Nogai Khan, his first name actually Isa (Jesus), was a vassal to the Crimean Khan Islam Girey I. However, he wielded such power and influence, that he was effectively a khan-maker, or king-maker, behind the scenes. This is why it makes sense that Nogai is only true political rival/claimant against the Crimean Khanate faction leader. My girlfriend is of the Nogai clan. I'm from the Naiman clan by paternal line, Argyn/Argun clan by maternal line