hi . the first thing is there is no right or wrong projection to use.What you are trying to achieve is an unwrapped 2d map of the uv's so you can make a 2d texture to fit , so the thing to do is find the best workflow that suits you for instance some people might use a spherical map to do a head where others might use a cylindrical and maybe a planar also.
You need to map the objects the best you can and then you will most likely have to manually unwrap the remaining overlapping edges in the uv texture editor.
The point about the stretching of your numbered texture is yes it will affect the final texture since it is a reference of how your texture will fit on your model ,so stretching on the numbers means stretching in the same place when you apply your texture.
With regards to making seamless joins the best way is to move and sew the UV'S as much as possible , of course there are cases where you will not be able to sew the maps so what you try and do is either hide the join by mapping so the join is somwhere on the model that might not be seen i.e the back of a leg or under an arm ect or you might be able to align them in the 1 to 1 texture space but not necessarily sew them but it will give you a reasonable chance when creating your texture to align the seam.

hope this helps you a little but it does take practice and a lot of patience.
regards J.S.