Not really. I usually refrain from labeling fictional character unless it's 'into your face' type of mental health issues (such as Dazai's depression or Atsushi's PTSD). Especially when it comes to side characters without enough screentime for any deduction (please don't take this as me telling people what to HC, this is just me preferring to stick to canon as close as possible). With all that said I honestly don't know if I'd give Fyodor anything, at least that I know of. As for Nikolai his entire character can be an allegory for mental illness (caged mind that envies other for it can never be free) so I'm sure there's at least some mental disorder that would fit him. I've seen people say he has ADHD and/or BPD but again I'm neither a psychiatrist nor know about either well enough to agree or disprove