I don't see why not. He clearly performed better against SSJ Goku than his Mega Rild form did, plus Goku did comment on it being better iirc. You may want to recheck the GT Strength Checker to confirm that though.
Just checked the fight out. There isn't much hand to hand combat; a lot of Metal Rild's screen time is him using his weird metal abilities to try and trick Goku and turn him into metal with his metal beam. The only time Rild actually tries fighting, Goku dodges all of his hits and sends him flying with a kick. Rild then lands one surprise blow.
It would seem that Super form is just the Metal form.
Episode 20
Time: Roughly 12m30s
Context: After successfully hitting Super Saiyan Goku, whilst in his Super Hyper Mega form
Rild: “It seems you've greatly underestimated Rild-sama.”
As you've guessed by now, that's indeed his Metal form, whilst Hyper Mega Rild is his Sigma form. It's what they're named in the Japanese version by the narrator.
I always figured it was something to do with M2 and not the Sigmas. He claims to be linked to the planet, and I don't see why Rild would be limited to only using it after merging with the others. So I'd say no for needing to save it for the last stage of his Sigmas transformation.