Hi. 5! Wow. You might find help on the Fostering or Adoption boards, probably the Fostering one for you.
What you need might depend on why you are becoming carers (eg illness/death versus neglect abuse.
When we adopted one thing we had to make clear to 8yo was that we were the parents of the 2yo, we made the decisions, not her.
The 3&5 will need very different rules from the 11,14,16.
Older ones: Rules about level of freedom, what they have to ask, and what they can just inform (eg being in for meals, friends round etc). Being contactable by phone. Doing schoolwork, bedtimes / being up for school without fuss. What swearing is allowed. Politeness. Parenting by consent especially for eldest. (bedtime 8:30-11pm)
Younger: Eating at table, kind hands&feet, sharing, felt-tips at table, look after property, no throwing anything inside house. (bedtime 7pm)
Hope you have more than 1 bathroom.