How to Make It
Prepare the Slaw: Whisk together mayonnaise, vinegar, sugar, salt, and cayenne in a large bowl. Add both cabbages and cilantro and toss to combine. Let stand at least 20 minutes.
Prepare the Tacos: Pour oil into a large heavy-bottomed pot to a depth of about 2 inches and afix with a deep-fry thermometer; heat over medium-high to 350°F. Combine flour, cornstarch, and salt in a medium bowl. Make a well in the center and whisk in seltzer and egg white until just combined.
Dredge half the shrimp in batter, shaking off excess. Carefully lower into hot oil and fry, turning occasionally, until golden and cooked through, 2 to 3 minutes. Transfer to a paper towel-lined plate to drain. Repeat with remaining shrimp.
Divide shrimp evenly among tortillas and top with slaw, hot sauce, and scallions. Serve with lime wedges.