I agree. Especially since part of their story is the fact that Benedict spent the two year period after their initial meeting hoping to find her and just begun to believe he never would when he re-met Sophie so that his lingering but muted dream of the woman from the ball informed their relationship.
Likewise, by the time they do Benedict's book, they should introduce Francesca's first husband if they plan to follow her book.
As for combining stories, It might work. Romancing, To Sir Philip, and When He was Wicked occur in overlapping time. So, if they don't combine them, the timing will have to be shifted or it will appear the siblings live in separate worlds. Or, they could do them like the books and simply have three seasons in which, while following only one story, the elements of the simultaneous stories are in the background.
On the upside, about 5 years elapse between Benedict's book and the three above. Gregory is just out of school and by this time so he should be an adult... or if they decided to have the adaptations take place sequentially rather than simultaneously, time period, he could be away in school to avoid addressing his aging from teen to adult by having him away at school. He really only figures into To Sir anyway and mostly for Eloise to mock his 22 year old self for trying to imply he has some role as a protective brother.