Update: I fitted the FPR ultra flex nib on the ADMOK M600 and it seemed to work. I had to used the feed from the ADMOK at the moment but I'll test out other pens later.

Products mentioned in the video:
Pilot Prera Iro-Ai [CM] - Pilot Iroshizuku Yama-budo
London Pen Company Christopher 14 [Bock - Stub] - Wearingeul The Great Saint Heaven's Equal
Nahvalur Original Plus Lovina Graphite [Stub] - Colorverse Blue Dragon Glistening Gold
Pelikan M200 Brown-marbled [IB] - Herbin Terra de Feu
Pilot Custom 742 [SF] - Diamine Chopin
Fountain Pen Revolution Himalaya GT Amethyst Purple [Ultra Flex] - Sailor Manyo Nekoyamagi
PenBBS 480 [Blade F] - Wearingeul I am a Cat
Hongdian M2 Gun Metal [Long Blade (from Black Forest 1861)] - Sailor Manyo Haha
Sailor Compass Fude De Mannen Dark Blue [Fude] - Sailor Souboku