You can mill with semi chisel chain, full comp, skip, or full skip, or use milling/ripping chain. It depends upon how big the wood is, how powerful your saw is and whether softwood or hardwood. I would suggest starting with full comp semi chisel. It can be resharpened to milling chain.
As for mill size, again, it depends upon the size of wood to be milled. The 36" Alaskan will yield about 34" maximum cut with a 42" bar. Add in the fact that a 30" log will yield about 21" square cant(70%). You can still mill larger logs, but may require some added steps. With the addition of a Granberg Mini Mill, you could make a top cut with the Alaskan, and then square the sides with the Mini Mill, and then even make a vertical center cut to split the log in two.
A 660 is a nice saw. It has an excellent power to weight ratio, and will do fine on the mill. Stihl lists bars up to 36", but Oregon makes a 42" that will fit. 3/8" pitch chain is a good size for the 660, coupled with a 7 pin rim sprocket. That 42" Oregon bar is .063 gauge, and Stihl makes 16"-36" bars in both .050" and .063".
I bought the 24" mill to start, and added 36" rails later. My inital though was that I wasn't likely to encounter much wood bigger than 24". After milling with both 24" rails, and 36" adjusted to 30", I find the smaller 24" far more handy. I felt like I needed help when using a 36" bar on 30" wood.
With bars longer than 30" it might be a good idea to have the auxiliary oiler. Stihl does make a higher output oil pump for the 660, but I'm not so sure that the standard pump and axuiliary oiler wouldn't be better. I seem to get enough oil to my 36" bar and chain, but think a little more would be better.
It would be a good idea to run at least several tanks through a new saw prior to putting it on the mill. Milling is tough duty for the saw. Likewise, using a mix ratio lower than 50-1. I've used 40-1 and like it. Also, richening the high speed mixture will help the saw run cooler. I've seen reference to tuning back from 12,500 to 11,500 for milling.
18" boards will cut much quicker than 30" boards, and still look impressive.
As for cedar, if its rot and bug resistant, you could use it for most everything in a cabin.
Have fun and be safe!