The plant needs both in the right concentration and ratio with other nutes given. This need to supplement what is in your soil. So, there is a lot of trial and error that goes into this.
take notes, because these things occur over weeks and months once have have a half-decent formula/method and a plan for adjusting it over time -- ramping up, maybe adding more of 1 thing than another.. just depends on what comes in the soil to start.
so, stick to a consistent dosage along with the rest of your nutes, if you see symptoms over time, you know a little adjustment is needed.
how much calcium and magnesium (these are 2 mutually exclusive things relative to plant needs) do the other parts of your fertilizer add? if none, then yeah you'll probably have to add some amount of calmag each feed.
molasses may add a total of 68ppm but most of that is not used by the plant. some bits are, but i don't think it adds a large amount of any 1 thing. the sugars will help feed any useful bacteria, but may also draw attention of ants, so keep an eye out.