Sparring works as long as you got people with good sparring worth in the sessions. There's 2 theories on the size of the sessions with some thinking you can have 5 at most before it gets divided and some feel you can have 9 people. Basically what happens is if you go over the cap, the people in the sessions will get divided into groups who spar with each other. So for instance if the Sparring Size Cap was 5 people, you could have 5 people in the class and they will all spar together and you'll get value for everyone, but if you have 6, they will be put into 2 groups of 3. The highest 3 people in terms of Sparring Worth will go together while the lowest 3 will go together, and so on.
Personally, I go with the 9 theory just cos I got a bunch of guys who need it
So I have some fighters on my roster who are purely there to improve my sparring sessions and help my younger guys improve their Primaries through sparring (These fighters are called Spar Bots).
As far as when your fighter is ready to Spar. Personally I don't spar until my fighter's Secondaries are at the higher levels, just cos sparring will add unnecessary points and so I want them to have their skills already up there before points get wasted, since it will be harder to get them up with their Skill Cap being nearly done.
The one Sparring I will allow my younger guys to do is Boxing. Sparring something will also give small boosts to their related secondaries, like so:
Boxing - Punches, Striking Defense, Clinchwork
Muay Thai - Punches, Kicks, Elbows, Knees, Clinchwork, Striking Defense
Wrestling - Clinchwork, Takedowns, Takedown Defense, Ground and Pound, Defensive Grappling, Transitions, Escapes
Brazilian Jiu-Jitsu - Submissions, Defensive Grappling, Transitions, Escapes
*Note, may be additional in BJJ, not 100% sure*
So from the build I put above, Boxing will give you points in Punches, Striking Defense and Clinchwork - Skills I want my fighter to have. The rest meanwhile have skills I don't necessarily want my fighter to have. So I am okay with my younger guys doing Boxing sparring every so often, but I hold off on them doing Sparring the others until they are older. Also, I don't have them spar BJJ at all really since by just training their Defensive Grappling and Transitions, their BJJ will go up to Remarkable / Wonderful which gives them a Brown Belt which is good enough.