You can't use 75/25 sorry to say, it sure would make things simpler (and cheaper) if the mix was that versatile but unfortunately it's not.
From the looks of the welds in that pic a tad more heat would probably help and some pre-heat would help also, especially on heavier stock. The reason the weld does not start out very good is, as ericnova pointed out, the material is sinking off the heat too fast and as it becomes hotter from the weld the bead will smooth out, pre-heating would solve this but just welding a little hotter would make a big difference too. Clean throughly with a stainless steel brush and make SURE the brush has never been used for anything else. A common mistake is to grab up a brush that has been used for various other chores thinking because it is stainless it will be fine but if it is contaminated it could easily cause problems, don't use your Aluminum cleaning brush on carbon steel (especially important when TIG welding and critical when using Oxy/Acetylene). Those sanding discs New Interiors mentioned are also an excellent way to clean Aluminum and are my first choice unless a brush is the only thing practical to reach into the joint.