I think the RO water will do fine, even without the CalMag+... It would definitely be BETTER with it, but it wont be bad.

I would be REALLY careful about your watering since you have such a tiny plant in such a huge container... a solo cup will support a plant just fine until its about 5-6 inches high. You probably wont want to soak your soil or there will be water just sorta sitting around in your pots instead of being used up by the roots. this means higher risk of root rot and mold and shit. be careful.