Oh, I'm very familiar with the 8411's. I used to use them on everything and I just rebuilt them and use them for throttle servos on my DA170 and Da200 engines. The will be just about perfect at 6v on the size of your plane, but it's actually more about weight that size when it comes to servos

As far as the servo arm screws. I have 3 different Ace hardware stores where I live and all of them have the screws in allen head form in the isle where they sell individual screws/nuts/bolts (I can it the Yellow Box isle) it's the isle that has all the little plastic yellow boxes in it. Home depot and Lowes cary them as well but metric is a hit or miss thing and Lowes and Home Depot are more expensive than Ace Hardware. I think I paid about 10 cents each the last time I bought them and I bought 25 of them so I could have spares.

I forget the exact size but just take one of the original screws with you and match the threads. You can go about 1mm longer and you will probably have to do that as the original screws are quite short but the 8411's can handle the extra length