Are you planning to purchase Disney GCs with the Target GCs? If so, the denomination of Target GCs doesn't matter. And they only sell Disney GCs in $25 and $50 denominations, so any Target GC with a value in multiples of 25 will work (ie $50, $75, $150, etc.). You just cannot exceed $300 in total value of Target GCs in a single visit.
An email alone is not sufficient to redeem a GC purchased at Target .com in-store. You will need to get the mobile version of the Target GCs so that the cashier can scan the code. There are instructions for redeeming included in the GC email.