In real use, there definitely exists a nuance between using perfect and imperfect in such cases.
"Не опоздайте" (perf.) is sort of bit less polite reminder to not be late than "не опаздывайте" (imperf.), even if both of those are used with "пожалуйста". "Less polite" because it sort of implies the possibility of persons addressed to be late.
This nuance, however, vanishes in less formal conversation.
Pronoun isn't usually used in such phrases, just "не опаздывайте к обеду, пожалуйста" would do. Names, however, could be used.
Pronoun with full stop (written with comma) is right out, that's an equivalent to "hey you", just like @user31264 says.