Important parts of a squad is always
1. Dealers of Damage
Those who dish out the damage.
Those can include characters with various weapons, special gunners and heavy gunners.
2. Absorbers of Abuse.
Those who take the damage so that your Dealers of Damage don't take any damage in return.
Depending on how big the guns are on the Dealers of Damage section and how tough the Absorbers of Abuse are and what kind of roles they're supposed to fulfill the the ratios can vary immensly.
Everything from Cannonfodder Grots where you at max put some equipment on the Slaver....and there are 30 grots....to Standoff Broadsides, Obliterators who are pretty tough themselves and really don't need no AoAs.
When it comes to sisters it varies between a 5-1 or higher for front line Reds to 2-1 for Retributors or Suicide Flamer squads.
Not even in a HQ should you have more characters and special gunners than you have regular celestines.