Mix the fruit rinds and juice, oil, chilli, garlic, sugar, cumin and a little seasoning together in a shallow china or glass dish. Add the chicken and toss together well. Cover with clingfilm, marinate in the refrigerator for 2 hours (or longer if you have time).
When ready to serve, thread the chicken on to 8 wooden or thin metal skewers. Cook under a hot grill, turning several times for 10–12 minutes until browned and cooked through.
Meanwhile, to make the guacamole, halve and stone the avocados and scoop the flesh out of the skin with a spoon. Roughly mash on a plate or in a food processor with the lime rind and juice. Season and stir in half the coriander.
Serve the skewers on plates garnished with the remaining coriander, lemon wedges, spoonfuls of guacamole and small bowls of salad and roasted sweet potatoes.