I had a swivelling Vertical Slide with my ML7. It was a bit limiting, and did not always stay where it had been set. It was the early single bolt version; later ones used two clamping bolts.
Then I bought a Rodney Milling Attachment. A nice piece of kit, but maybe having been used to Cincinattis, maybe I was heavy handed, because the ML7 was not rigid enough for even light cuts (0.010 – 0.020"
with an end mill.
For a particular job that would not fit on my Warco Economy Mill/Drill, extravagantly, I bought a V S intended for a Seig C6, and made adaptors to fit it to my Engineers Tool Room BL12/24 (Warco BH600 or Chester Craftsman in a different livery).
The C6 slide locates on a 10mm dowel, where my Front Toolpost locates on a 25mm one. Simple, just turn up a suitable 25mm bung with a 10mm reamed hole, with a M6 tapping at the bottom to allow retrieval. This neatly coincided with the angular graduations on the Cross Slide,which was a bonus
To be really masochistic, I then made up an adaptor to allow it to swivel in place of my four way back toolpost. Graduating that was bit of a pain, especially as found that the original graduations were inaccessible. (WHO said Plan Ahead?)
So after all this ramble, you CAN mill in the lathe, (LOTS do, very successfully) but it will not be as rigid as a purpose built machine, so don't be like me, overambitious. And DO use a good vice on it to hold the work.
Howard