Rancid coolant is caused by bacteria that multiply when there's no air in the coolant and it can stink and cause dermatitis. A small fish tank type air pump slows them down. Synthetic coolant seems to be less prone, but others will have to confirm actual type.
The 4th axis is basicaly a CNC controlled rotary table- it should have T-slots to bolt things on to.
A tailstock is used on a lathe (you have a mill!) to essentialy hold the end of bars that you want to machine- without it bars or rods tend to wobble so much that they fly out of the chuck (usualy at head height). If the chuck is on the left, the tailstock would be on the right, past the toolpost. You usualy drill a small v shaped hole in the end of the bar with a centre drill then put a live (or dead) centre in the tailstock and push it into the v hole to hold the end of the rod.
I use a 3 jaw chuck on the mill to hold circular parts (I clamp the chuck to the mill bed)- it's handy if you're doing lots of the same part. For a square part you'd need a 4 jaw chuck, but youd usualy use a vise.
I'm not an expert though! so please feel free to add or ammend anyone!
Before you go buy cutting tools, most folk recommend HSS, two fluted end mills for ally. Carbide doesn't like thermal shock from the coolant and can chip.