Red on stems can be indicative of Phosphorous deficiency... I don't think it is associated with Mg at all. But it is more likely to be a quirk of the strain, & since you've seen it from the get-go and no other signs of P deficiency (red/purpling on leaves, slow growth), I think you're right not to worry about it.

As MisterE said, most plants merely double (rather than triple) in size during flower. You can either figure on triple to be safe, or assume double and LST if they get too tall.

Lookin' good!