To avoid solder drop out you can heat the fitting at the top of the top cup, control your heat well and balanced, put a slight working curl on the solder so you can write with it fully around the joint. Then carefully add just enough solder to the bottom joint to fill it by slightly going all the way around the bottom perimeter and controlled feeding in your solder. Then you quickly transfer your solder feed into the top cup. You should always wear a nice clean pair of cotton gloves so that if it does run or form a sharp you can gently wipe the offending slobber spot.
I use Nokorode which leaves a mild wax feel, so I tend to use a scotch brite to polish up the cooled joint. If I'm doing a full repipe I'll use a rag to wipe it while it's still warm to get the wax off.
Soldering is a lot like pinstripping, you get a feel and it just happens, after practicing and developing your technique.
A neat trick is to unroll about 10 inches of solder, put a curl on about 4 " of that, hold the roll in your palm and feed out the working curl so you are feeding it with your pointer finger. You will be holding the solder roll in the holes, sort of with your thumb and pinky. By feeling the feed with your finger you can feel any debris, dirt or souring of the joint, it feels gritty, like sand while a good flow slides nice and unrestricted. And you should always circle the entire joint with the solder so as to cap it.
On such small diameter air, relatively low pressure, I don't really think that a lot of physics will factor in. Air tends not to react the same as water and deteriorate pipe surfaces in restriction spaces at lower pressures like we would use so intermitently in a shop. Not to say that it can not, just not in a home shop or small system as you are describing.
I have set up a few aquarium shops in which a Gast centrifugal blower is used or an oil less rotary blower, very high flow rate low pressure system, the whole system may push out 20 PSI sustainable, yet operate as many as 1000 air outlets or more each exiting under a water column of 18 to 24". If we added the combined pressure needed to aerate the system by total load the numbers would be astronomical as to the pressure required but since we are high air flow we actually use the weight of the air in motion as a force to overcome the pressure needed.
By doing this we keep down heat and power needed to operate the unit. This unit typically requires a 5 amp power source. Even under such a high flow requirement as this you get no residual deterioration of components nor any real pressure loss due to bends, even hard ones.
The pump basically forces a flow that feels much like a very high performance vacuum discharge, but it has the capability to step up it's capacity as it is taxed. It's like a little turbo.
The bad thing about the copper jointers is, they should only be used once and then discarded, same as the gas appliance connectors. When guys reuse them and buy beer with parts money bad stuff happens.
