I have mach3 licensed with a sherline lathe, 2 kelling 4030, and 2 425 oz motors everything works great and couldn't be happier. Also: I have a sherline 5400 CNC ready mill with no motors or micro steppers at all. However: the X,Y, Z screws are all 20 TPI. My question is can I just get 1 more 425oz motor and another kelling 4030 leave the new one hooked up to my mill, and use my other stuff from my lathe and place them on my mill?

No reason why not. 

(I use 3 different machines, only one at a time, from the same PC by selecting different Mach3 profiles - this method of operation complies with my single licence terms).

Tweakie.

425oz-in motors on a Sherline? Wow, that is serious overkill and detrimental to performance. With the stock Sherline lead screws, which are 20 turns per inch a stepper motor in the 150 oz-in to 200 oz-in range is a much better fit. The smaller motors lower rotating mass and lower inductance better match the large amount of mechanical reduction from the lead screw.

To you question abut running two different machines. Mach stores all the machine settings in 'profiles' you can set up a profile for your lathe and mill, and then use Mach Loader to start up Mach3 with the profile you want to use.

Thanks guys! And yeah I found out the hard way about the 425oz motor ordered the first thing I seen.