I'd stay with an 18". 51cc is smallish for a 20" bar, but if you need 20" and don't want another saw then it will work.

Baileys often has decent combos. You might consider a hard tip bar (no sprocket). They are a little more durable but also have a little more friction (more reason to use an 18" or even 16" bar with a hard tip).

If your current setup (.325) is working and not worn, it's probably not worth the money to change. If you are having a hard time finding .325 chain and sprockets, that would also be a reason to change. Other than that, I don't think you'd see much change from switching.

I've always read good things about ripping chains, but I mostly read and rarely mill. I did not use a ripping chain, but would if I did it more regularly. I do think your saw is too small for milling. You can do it but it will really work it. I've milled with a 65cc saw and I wouldn't want to do more than 30 minutes with it again. Chainsaw milling is alot about horsepower and lubrication. I'd like to have a bandsaw mill but the cost is just too high.

Have to agree with Mas a 50cc Class saw is rather small for any serious Milling . Most saws that I have worked on which were Milling able were in the 80 cc class with either 36 " or 44 " solid Nose Bars. Oregon and Granberg both offer fine Semi and Full Skip Chains in 0.50 and 3/8 " Pitch . Most Oregon filing is set at 20 degrees with the Granberg set at 10 Degree as compared to Normal Wood cutting angles of 30 Degree to 60 Degree with Hardwood and Pulp wood cutting Limits . For limited use ptgr as you have indicated your Husky could be tried , beware that milling is extemely hard on any saw . Ensure to use quality Premium bar oil and set the flow heavy and set your chain tension taunt 3/16 " deflection at mid bar with 10 lb pull.