I have seen a large range of Art specialist in play, many of which have to be developed into during play. While I know most of the long time players with have experience with many of these, some might be new and overall it would be useful for beginners. I have tried to keep to the broad categories rather than the narrow focuses. Please add any I miss and any discussion on the advantages/disadvantages of each type.
The Single Form Specialist
These have one of the narrowest range of capabilities and will often develop into one of the more rounded types. Generally only found in new Magi who want to have a variety of spells using different techniques and a single form. They can start with a single Form at 15 without virtues. Also the narrow range means that Virtue selection is fairly easy, often along the lines of Major Magical Focus, Affinity with Art, and Puissant with Art. Thus they have free Virtues to spend on other areas that interest them.
The Single Technique Specialist
These have a slightly wider range of abilities compared to the single Form specialist, thanks to there being twice as many Forms which gives them a wider range of spells to choose from. They have much the same range of possible choices for Virtues.
Single TeFo Specialist
Probably the most common starter specialist, they can start with a pair of Technique and Form each at 10 without virtues. Within their combination they can actually start with higher level spells than the single Art specialist, along with having slightly weaker coverage of the spells of each type. For Virtues it is possible for them to go with a Minor Magical Focus if they wish to be extremely narrowly focused.
The Pre-Generalist
Due to the way character creation vs in-game XP works, players who wish to develop into Generalist are advantaged to start with their initial Art XP in ether a single Art or a TeFo combination. With a decent starter library they can get their other Arts up to 5 or 6 in less than four years.
The Three Art Specialist
Focus on a TeTeFo or TeFoFo group, I have mostly seen these form from a single Art specialist. This group is made of of Magi with a single high Art of one type and two moderately high Arts of the other type. Virtues for a new Magi of this type are often closer to the selection from a Single Art specialist compared to a TeFo Specialist.
The Double TeFo Specialist
Commonly I have seen these with a single high TeFo combination and a second combination that supports it. The second set often cannot be easily supported by Virtues.
The Form and Techniques Specialist
A growth from a single Form Specialist, this style has a single very high form and tries to raise all of their Techniques. In their specialist Form they are powerful across the full range and they tend to eventually have a decent ability with all other combinations. They do suffer from weak Magic Resistance outside of their chosen Form.
The Technique Specialist
A five Art specialist focused on the Techniques. There is not many Virtues that are useful across the range and they in general have weak Magic Resistance. However this is the cheapest path to having a decent score in any combination. They are not however a true Generalist since they are limiting the Arts they are putting points into to a third of the total. Magi who stay on this path and reach old age will normally develop into a Technique Generalist.
The Craft Specialist
While similar to the first three in Art selection and Virtues, this type normally focuses on Rego and spends a portion of their Virtues on ensuring that they can develop a high Finesse Ability. Because of their high Rego and Finesse they can be surprisingly effective in many situations beyond building/crafting things.
The Creation Specialist
This is similar to the Craft Specialist. However they focus on Creo rather than Rego. A high Finesse is still important for them and what separates them from a Magi who just focuses on Creo. While creating permanent items requires Vis, this Specialist can also create complex objects with a shorter duration.
The Magical Focus Specialist
This is a type built around a Magical Focus. Depending on what that Focus is, their Arts development might range from a single Form or TeFo combination to a group of five or six.
The Spont Caster
This type is defined more by their Virtues than their Arts. Virtues like Diedne Magic, Life Linked Spontaneous Magic, Spell Improvisation, Withstand Casting, etc.
The Elementalist
This type received a nice boost with the rewrite of their Virtue. This is a four Form specialist which receives a good boost in XP across each other.
And a type of Generalist...
The Technique Generalist
Since I mentioned them in the Technique Specialist I thought I would include them on the list. This is a Generalist who spends 1/2 to 2/3 of their time studying the Techniques. This sacrifices some Magic Resistance for the ability to build up higher castings totals across all the TeFo combinations early on. Effectively a Generalist who tries to take a slight advantage in developing their abilities from a Specialist mindset.