Many other Janome dealers will tell you there is no problem with using other thread types .... and needles!
I do not have a J7700, but did have a 6600, a similar machine of the same vintage.
My dealer had a whole array of threads that they sold, and recommended for the machine.
Cottons, polys, rayons and blends.
Likewise, they never pushed Janome brand needles.
More so, recommended Schmeck needles because of the broad assortment of sizes/types available.
There are a lot more sizes/types of needles out there than Janome offers.
Does it not make sense to use the "right" one for the job it will be doing.
The important part is that it be a quality brand ...