I have something similar, a Morso Squirrel converted to oil (don't ask

) you need to consult the stove manufacturer as to what size flue they recommend. Mine is only 4" any larger and it won't work properly. A friend of mine had a similar stove and used the existing large 8" flue, it took him about six years to work out why it never worked properly

Flexible stainless liner is definitely the way to go.
Good luck, Paul