All of the spells I've seen in the main rules that use either Ring or Circle, use both. Is it possible to have a Ring spell that affects an Individual or other not-a-circle target?
Likewise, a magi who learns Hermetic Geometry gains the duration Arcane Ring and target Arcane Circle, both of which are one magnitude higher than Ring or Circle. Is it possible to make spells that use Arcane Ring but only the normal circle? If a character has Flexible Formulaic Magic, Hermetic Geometry, and knows a Ring/Circle spell, can he cast a +1 magnitude version of that Ring/Circle spell, using one of the Geometry options, but not both?
It is perfectly possible to have spells that use Ring or Circle but not both. A Ring/Individual spell is normally inferior to a Ring/Circle spell, since it has all the drawbacks of the Circle target built in in the Ring duration anyway (the target must be in a circle drawn at the time of casting and the spell end if the circle is broken). However a Sun/Circle or Moon/Circle spell can be useful, since the target can leave (carefully, so as not to break it) the circle and the spell will not end.
That goes for Arcane Circle and Arcane Ring as well: usually it is advantageous to pair the two, because they share most of the same drawbacks anyway, but it is not an obligation.
Also Ring/individual spells could be useful for creo spells the create things since you have to use an individual or group target for those sorts of spells. So a CrIg spells with target Individual and duration Ring could create an eternal flame for instance.