Question
How much epsom salt should I use per square inch? Could I start the grass out using epsom salt? what about using it with other fertalizers?

Answer
Epsom salt is also called magnesium sulfate which means that this fertilizer contain two specific nutrients: magnesium and sulfur which is (also) used by grass.

However, espom salt does not contain the macro nutrients which grass most often needs to grow: nitrogen, potassium and phosphorous. These 3 (key) ingredients in fertilizers are used in large quantities by grass plants and therefore you often find these nutrients packaged in fertilizers.

If you take a look at a lawn fertilizer package, it may contain numbers such as 29-3-4, 10-2-6, 33-3-3, etc. These numbers stand for percentage of nutrients in the bag of N-P-K (Nitrogen(N) - Phosphorous(P) - Potassium(K)). E.g. a 29-3-4 bag will contain 29% nitrogen, 3 % phosphorous, and 4% potassium (always in this order: N-P-K).

The reason we fertilize lawns with fertilizers which contain nitrogen, phosphorous and potassium is that these nutrients are consumed fast by the lawn grasses and often need to be resupplied (can be depleted from the soil).

Unless  you have a specific nutrient deficiency, it is normally not required to add magnesium or sulfate to the lawn seperately. magnesium is typically readily available in most soils, although some soils are naturally deficient.  Sulfate is seldom missing.

Therefore, unless a soil test have shown that your soil is deficient in magnesium (or sulfate, which is however unlikely) then there is no reason to add epsom salt (magnesium sulfate). Epsom salt have no "magical properities" beyond the supply of magnesium and sulfate and unless a soil test show that your lawn require these nutrients, then do not add.

If you do decide to add epsom salt to the lawn, then use very sparingly as the salt contents can kill lawn grasses if applied to heavily.