1. For a party about 1 person (who loves music) should i do a lot of mixing? Or should i just play tunes.
You a DJ not a jukebox, show off your DJ skills !
2. Should i add songs that he may not necessarily heard of, but may like?
Experiment with it, for example throw some Martin Garrix – Animals, and see the reaction and be prepared to chop it short if they throw you with beer bootles (lol jk)
3. If i get some music that he likes from family, how should i space out his favorites?
Instead just get it from his family, get it directly from him also,
4. Is it ok to add some new more modern songs into a playlist optimized from the 80’s??
Sure !, although dont just limit yourself from 80’s tunes, throw some 70’s and some 90’s also