Well, considering the lore of the game already states Alchemists are healers (the ALC Guild doubles as a hospital), even though gameplay wise they are specifically Disciples of the Hand and thus have no use in battle, it still makes a hypothetical 'Chemist' healing class confusing in terms of title - but a slight name change would probably go a long way to solving that.

As for the OP's ideas, the 'Fusilier' class is somewhat redundant - why not just have Musketeer dual wield guns and thus role the two classes in together? In fact, the class icon for Musketeer that's been in the game in the Coral Tower since 1.0 actually shows dual pistols, and there are numerous npc MSKs in the game who dual wield flintlocks - Merlwyb is the obvious example.