I started to make segmented pens David because i had lots of cut off's, as i hate throwing anything away. You will find as long as all the cut off's have squared edges you can glue just about anything to anything. So one weekend i set about gluing up some cut off's, don't worry about pieces being bigger than others or how ugly the blank looks, as you can use a sander to square it off ready to drill, and once you start to to turn it suddenly it doesn't look as ugly ... When the final sanding and polish is done your ugly duckling becomes a beautiful looking pen ...
I guess my biggest questions are: do you plane the mating faces of the blanks before gluing?
No planning done at all, all you need to do is make sure that the two blanks that you decide to use have all the same diameters ...
What glue do you use?
I use medium CA on all my gluing, thin for filling in ...
When the mating faces run at an angle to the length of the pen blank how do you ensure the pieces don't slide out of position while the glue sets?
I cut the two blanks in half, and then use half of each blank for the cutting. A small drop of CA on the very edge of the bottom blank is enough to blanks both blanks while you do the cutting on either a bandsaw or scroll saw ...