I have Sugar, 3 lb, Spike 4lb and Gracie 9 lb. We don't have any problems as far as safety when they play and Spike and Gracie play pretty rough sometimes. I think Gracie knows to be careful with him. You are right though- it's a big difference in size, especially to carry around. Sugar and Spike will sit in you lap together but there's only room for 1 Gracie.
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226