In general, you only use 'elles' for an all-female group. For example: Il y a cinq filles dans la maison. Elles sont.... For mixed groups, you need to use the masculine plural pronoun 'ils': Il y a cinq filles et cinq garçons dans la maison. Ils sont... In fact, if there are five hundred girls and only one boy, you'd still have to use 'ils'! You are right about 'personnes,' though. 'Personne' is a grammatically feminine word, so the pronoun is feminine: Il y a dix personnes dans la maison. Elles sont... The same would apply to any grammatically feminine word. For example, the French word for 'victim' is 'la victime'. This is grammatically feminine, regardless of whether it refers to a man or a woman, and the pronouns which go with this word have to be feminine too. Note that this has no relation to the gender of the actual person. Feminine/masculine gender of words is not necessarily the same as sex or biological gender - it is a purely grammatical distinction. For example, a mouse is 'une souris', so all mice - whether male or female - are always grammatically feminine.

Ils. Only elle/elles when it is exclusively female

Ils. You only use ells when its only females.