Canonically the answer is no.
To be able to learn any hermetic art your gift must be opened to hermetic magic.
However if you analyze the extended ArM universe you will not, as I am sure you have, that many hedge traditions with TeFo type arts, e.g. the learned magicians and the Augustan brotherhood, have initiations into individual arts.
This begs the question, why are there no such initiations for hermetic magic?
The question has no answer in canon, so you will have to come up with one for yourself.
I think that the real reason is an offgame/metagame reason, namely that very few people want to play just a part of a hermetic mage. Whereas ungifted learned magician or augustans can be companions and they can use the partial initiations into their respective arts. Players playing gifted learned magicians will want to have all their learned arts opened at the same time in a similar manner to how hermetic magic is opened.
You could argue that the Order of hermes does not have such partial initiations because it is not open to the ungifted, but I dont think that the order is only open to the gifted. If memory serves house Ex Misc has a small number of ungifted members who joined because they have supernatural abilities and were presented with an offer of "join or die" and chose "well the one that isnt death".
If hermetic magic is not fundamentally different from say learned magic or augustan magic then initiations into individual arts could exist. So why dont they? Is hermetic magic fundamentally different from all other magic? I would say no. Maybe those initiations into individual arts exist somewhere but were never published in an existing splatbook. Maybe the Order of Hermes forbids development of such initiations, but it was never specified in a splatbook?
As you can probably see the true answer is that there is no definitive answer. you have to make your own.
However assuming you decide to initiate the arts individually like for certain hedge traditions you would be doing a gifted person a disservice. Such piecemeal initiation is decidedly inferior to the whole initiation that is only open to the gifted. But assuming you did so I would say that form bonuses do apply, I would also allow certamen but only in the opened forms.
You would probably be better off paying a hermetic mage with a high InVi total or even a pralixian to open the gift of said gifted companion.