Place the skirt steaks in a nonreactive baking dish in a single layer. Combine the chili powder, coriander, salt, cumin, garlic and onion powders, and black pepper in a small bowl and mix well. Sprinkle this mixture over the meat, patting it on with your fingertips. Let stand for 10 minutes. Squeeze the limes over the meat. Let marinate in the refrigerator for 30 minutes to 1 hour, covered.
Place the poblano and bell peppers on the hot grate and grill until charred on all sides, turning with tongs, 15 to 20 minutes. Transfer peppers to a cutting board, wrap in wet paper towels, and let cool. Meanwhile, grill the scallions until nicely browned, 3 to 5 minutes per side, turning with tongs.
Unwrap the peppers, pull off the burnt skin with your fingers (or scrape it off with a knife), cut the flesh off the core, scrape out the seeds, and cut into ¼-inch strips. Cut the scallions into 2-inch pieces or keep whole.
Arrange the pepper strips and scallions on a platter. Place the cilantro, jalapeños, avocado, sour cream, salsas, and lime wedges in attractive bowls.
Place the tortillas on the hot grate and grill until soft and pliable, 15 seconds per side. Place them in a basket.
To serve, have each guest place sliced beef and grilled vegetables on a tortilla and spoon some of the cilantro, jalapeños, avocado, sour cream, and/or salsas on top. After a squeeze of lime juice is added, the tortilla can be rolled into a neat bundle for eating. Tying the tortilla with a whole grilled scallion makes an elegant touch.