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Hi, I'm Mike G and I'll be assisting you. You can install a 100A main for the sub panel as long as information on the main service panel allows it. Check the label. I suggest seoerat GFCI breakers for each branch cirvuit, not having one tripping for any function. Easier to troubleshoot if necessary. You might even consider a 200A sub panel with a lug kit, making your existing main breaker the main for the sub panel.
200A sub feed can be fed with 4/0 aluminum SER cable. As suggested individual GFCI breakers for each circuit.