I have Espoma's Organic Garden Lime, which does contain magnesium.
Two questions..at what rate should it be added and is additional epsom salt necessary?
IMO additional epsom salts are not necessary, and the amount of lime is 1 tablespoon per gallon of 511 mix.
Note that the garden lime is being used here to adjust the pH up slightly. It will provide some Ca and Mg but likely not enough and not in the correct proportion for Ca hungry tomatoes (unless you have high levels of Ca in your water or something).
I'd make sure whatever fertilizer you use has Ca and Mg in something like a 4:1 ratio or find something like Cal-Mag to add to a fertilizer that doesn't have them (or make your own with calcium nitrate and magnesium nitrate).