For what occasion can you use paper napkins?
You can use paper napkins for all occasions. Of course, by taking account the theme of the occasion, you can change patterns and shapes. If you use gold paper napkins with an art deco motif, you're making it clear that it's Christmas or a Gatsby party ! If you use striped paper napkins and you choose a mix of colours, it's very likely that you're organising a picnic... The combinations are infinite.
Can you mix colours, patterns and sizes of paper napkins?
Yes, just as you can mix sizes and patterns of paper plates. Some will mix very well together like stripes, polka-dots or diamonds, but that goes for liberty motifs, single napkins or shaped napkins too. What would you say to mixing paper napkins in the shape of tropical leaves, watermelons and lemons ? It's summery isn't it?