"These Swiss people are very friendly."
Translation:Những người Thuỵ Sĩ này rất thân thiện.
Both, but "thụy sĩ" is more popular. Besides, I've never seen the word/spelling "thuỵ sĩ".
Because "Indians" mean every indian, and "these Swiss people" means an specific group of people (imagine the speaker pointing at some group of people).
Classifiers and plural particles are only needed when speaking of specific beings.