I'm agree with Alex. With the correct dilution you can fix the eyes with acrilics. I saw Jaume Ortiz paint the blue eyes in 75mm in seven minutes (i stopwatch when he said he was going to paint the eyes with the whole face painted) and fixed the bigger round in a couple of minutes or less. I can't do it, but is possible. And i hate Jaume Ortiz for this
The trip is practice and a good brush (i use W&N serie 7 or Da Vinci). Jaume used a 0 Vallejo brush. He painted all the miniature with the same brush. And as Alex said, don't need a ver small brush. I do them with 0 or 1. Smaller brushes 00, 000, catch not painting and is impossible a fluid work. I do what i can with acrylics about eyes. Can't get decent eyes with oils. Easier with acrylics (all is easier with acrylics for me )