In short - yes.
I saw this and your other related question about your Dr No. Character. I played a similar Meta-magic specialist in Ars Magica a while back. I focused on fast-casting virtues, and I did indeed take Minor Magical Focus (but I focused on fast-casting, not counterspelling). I was able to cast counter-magic effectively but was also able to cast useful meta-magic like level-boosting other magi's casting, or changing attributes of spells as they were cast. If you are focusing purely on counter-spells, you should be able to create a very solid caster. My character turned out to be broken, however - be careful not to make your villain undefeatable.
I found that the only Form I needed was Vim, and used Techniques as follows:
- Creo - add power to allies spells.
- Rego - retarget spells
- Muto - Change Art in a spell
These two were the only spells I developed as formulaic, as it was too high-level to cast without. I mostly used this to mess with enemies' casting. Changing Creo Ignem to Creo Aquam for example, took a lot of the bite out of combat magi! I could also use it to make my allied casters more versatile. I still needed to be able to fast-cast this one, but it was easier to fast-cast a formulaic spell than to make it up on the fly.
- Perdo - de-power or counter enemy spells.
- Intellego - used to work out the Arts used in spells/effects that I wanted to mess with (not useful in combat or other rapid-fire scenarious, but in messing with wards or rituals or other lasting effects/magical creatures)
We found that I was able to counter anything cast by anyone of even remotely the same power-level as my character, and when not focused on the enemy, could boost the power level of my allies far more than the use of another magi. It was really hard for the Keeper to come up with anything we couldn't defeat without spamming the power-level (and ending up giving us no chance to win). So while this character is creatable, it comes with a caution!