If i don't start my bcp on sunday instead on monday. should i follow the 7 day rule?

Doesn't matter: You can start you pills any day of the week, it does not matter. There is nothing magic about the Sunday start - the only reason to start the pill on a Sunday is to lessen the likelihood of having your period on the weekend.

Yes: The 7 day rules is based on it taking about 7 days for the BCP hormone to put the ovaries to 'sleep'. If a pill is missed or taken a day late, the ovaries might have 'awakened'. keep taking the pill for 7 days without trusting it. Then the ovaries should be back to sleep.