Some questions regarding communions

[Humakty]

No idea on the three first, but :
4/ use and abuse if you've got cheap communion slaves (thinking of MA Pythium and MA Ermor). I never use one to cast lowish spells, so I start to field communion when I've got level 3/4 spells. It also depends on your mages picks : when playing Pythium, it is almost useless to field non-communioned mages, because their picks are spread among too many magic paths.
I forgot Marverni can also make usefull communion, but if I remember well, the stargazer is a bit costy (non sacred + high gold cost)