It is confirmed that his weapon is a catalyst from the latest leak
Scarmaouche shouldnt be a catalyst, he should have a sword or a bow. Lisa is an electrical catalyst so Scarmaouche would be just like her. No one has an electrical sword so I think that he should have that. I'm disappointed to see otherwise.
With his huge hat, it'd make sense for him to be a catalyst.
claymores, polearms, swords, and bows all have animations that use a very wide span of movement from the character since they have to hold it. a claymore or polearm realistically would get caught in the screen of his hat way too easily and with how the animations of other teen male users are having such a wide span of movement animation (Albedo, Bennett, Xingqiu, Kazuha and how they all swing behind them and over their head at some point) it just wouldn't make sense unless he quite literally cuts up the screen of his hat or knocks it off his head.
for a bow, the weapon itself is bigger than you think it is, and to hold it up properly to do a charged shot, the bow would most definitely clip into Scara's hat.
The weapon that moves the least is a catalyst. Note that the character can move as much as they'd like but they can control the catalyst to stay in relatively the same spot and Ningguang doesn't even show her weapon at all unless she uses her skill, burst or plunges but after will tp to behind her and disappear after a while). This gives Scara free space to do whatever he likes without his weapon colliding into anything and Mihoyo is very careful with animating everything so nothing clips together without glitching of some sort.
Those are my two cents on the possible weapon explainations