You will likely see extremely limited benefit from your preheating efforts unless the material is wet. While it is necessary to preheat many of the more uncommon, high strength Aluminums prior to and during welding, with the more common extruded, structural alloys (6061, 6063) it is not required. Heating it to just 150 or 200 will drive out the moisture which is the only real benefit you will realize with preheating those alloys. In other words - don't worry about the preheat - you are wasting your effort in my opinion.
Look into "Alumi-brite" - it is a phosphoric acid cleaning solution for weld prep. I only use it occaisionally for heavily oxididized materials. It works really well on copper, by the way. 99% of the time, I simply put a fresh bevel on the edges and weld and I weld a lot of Aluminum extrusions.
To help keep you filler rod still, try resting it in the joint - just ahead of the arc where it won't melt but still in the shield. When the base is ready, advance it into the puddle then back it off just a bit but leave it in contact with the work - this will help you keep it steady. You can get a rythym going and make a nice, even bead. Play with the torch 'attack' angle and the filler rod angle to find a comfortable system for how you like to advance. You are pushing, right?
ER4043 is the filler for 6061 and 6063. You are in for a rude awakening if you are planning to anodize, though. The weld anodizes so-so, the parent metal anodizes ok but the HAZ does not anodize hardly at all (6061 and 6063 don't anodize all that great to begin with). 5053 is a different story and if you are planning to anodize, you might want to use this "architectural" alloy with the appropriate filler.
Scott
Consistency is a good thing....unless you're consistently an idiot.