Ars 5e already heavily rewards specialization and one-note builds; a mechanic that provided (or supported) a reason to be more of a generalist would be welcome.
It's clearly not unbalanced if it applies to all magi. It would reduce their power. Put another way, that doesn't seem like a balanced virtue and flaw pair, as the flaw has a bigger scope than the virtue and bigger penalties than the virtue’s bonuses. Deleterious Circumstances specifies an “uncommon circumstance”, and 90% of magic isn't that! (If this affects all Hermetic magi, I don't think that's a problem.)
It would also have social consequences. It might be tempting to replace the Houses by one House for each Form. If you don't, there are consequences as (say) Corpus magi of different houses have things in common that they don't have with Ignem magi of their own house. It would mean that magi would have more of a reason to team up to fill their gaps in ability.
Would your best Form be inherited from your Parens? If so, each house would probably only support a few Forms.
And unless you write in some further rules, spells with Form requisites become very hard to work with.
In the issue of Sub Rosa concerning playing in different time periods (issue 16), they suggest as a rule for playing in 550AD that magi choose six arts (from a combination of techniques and forms) to represent their magical tradition in a world before Bonisagus’ unifying theories. This could work for the sort of heavily-focused on a theme magi you are looking for.