Choosing filler rod can almost be as bad as asking what 2 stroke oil to use
There are few different filler rods you could use but the short version is you will be okay with 5356
As to what alloy the arm might be is a good question, I have welded 6061 & 6060 components with 5356 and had them anodised without problems however I once repaired a early Suzuki arm with 5356 filler and also had it anodised however the bits that I welded came out in ugly dark blotches which suggests to me they aren't 6061
You will need to remove the anodising well clear of the section to be welded then your dilemma will be do you leave it like that or remove all the anodising so the whole arm looks the same. Any attempt to re anodise it will likely produce blotches as above so I would recommend a clear coat.
Also try not to put too much heat in as most structural alloys gain there strength from heat treating and too much heat will rob strength.
Use a heat gun to put some mild heat in then use high amps so you can get in quick to the area and avoid heat soaking the whole arm
