I believe DUP and Sinn Fein were excluded because they don't compete with the 7 represented. It's virtually a separate political system over in NI.
I don't think the seven split on the audience is right. Look at another way. Corbyn become prime minister with 350 MP's. Farage leads UKIP and is finally elected, the only UKIP MP. The next election debate, Farage has the same number of supporters present as Jez. Doesn't sound right to me
Seems a bit unfair too though in that scenario that Farage would have one supporter in the audience or none. Maybe base it on the vote share rather than number of MPs which would give fairly even split of left and right.