[pugs]

Not postive on the Cat 40 in a NMTB 40 spindle, might work, if the drive keys are same etc. I'll check the handbook and double check they are the same tapers etc.

As for holding smaller tools, typically use a collet holders like a Er-32 or ER-16 or TG100. I only use solid endmill holders for roughing tools as they engage the flat on the endmill (if it has one). The collets will hold more concentric which makes all the flutes take the same cuts resulting in smoother finish and more accurate size, as well as longer tool life as all the flutes are cutting.