Slice cucumbers in half lengthwise and then into half-moon shapes. Place in bowl with salt and let stand for 30 minutes. Drain liquid from cucumbers. Combine remaining ingredients and add to cucumbers. Cover and refrigerate for 12-24 hours.
Mix all ingredients and let stand for 15 minutes before using.
Rinse rice and let dry for 15 minutes. In medium saucepan, combine rice and 2 cups cold water and bring to a boil. Cover and cook over moderate heat for 10 minutes. Reduce heat to low and cook 10 more minutes. Remove from heat and let stand for 15 minutes.
In a small saucepan, combine vinegars, sugar, and salt. Warm over moderate heat until sugar is dissolved.
Mix the vinegar mixture with rice, separating rice while gently mixing. Transfer to a baking sheet and cover with a damp towel.
Combine tuna, sushi rice, cucumber kimchi, and sriracha aioli, and gently place on a tortilla.