Typically I think most of the big gyms go with a loose guideline like:
Level 1
No tryout requirement
During season: work towards walkovers
Level 2
At tryouts: standing BHS, roundoff BHS
During season: work towards specialty to running BHS series
Level 3
At tryouts: Jumps to standing BHS series or just standing BHS series (depending on routine), Roundoff BHS tuck
During season: work towards specialty to roundoff BHS tuck
Level 4
At tryouts: Standing Tuck, Standing BHS Tuck, Roundoff BHS Layout
During Season: work towards standing series to layout, specialty to Roundoff BHS Layout
Level 5
At tryouts: Triple Toe Tuck, Roundoff BHS Full
During Season: work towards standing series to Full, Standing Full, Specialty to Full, Double Full, etc.
Then depending on what is needed stunting wise and the caliber of talent that the gym is able to choose from, specific spots may have less tumbling requirements for the level they are placed (specifically flyers,back spots, or amazingly strong stunters). I'd say that for small teams it's pretty crucial for everyone on the team aside from maybe 1-2 girls to have the level requirement, as its very difficult to hide them during squad tumbling sequences such as standing tumbling. For large teams however, I'd say that you could have 4-5 that don't have the squad tumbling, as you could always have them do a crazy basket in the back while the rest of the team tumbles. Now of course depending on where you're competing,who you're against , and what percentage tumbling is worth on that score sheet will determine the importance of having 50% plus 1 with the level requirements vs. 80-100% with the requirements. At major competitions you can bet that big name gyms WILL max out. At regionals or small gym competitions however, you may be fine with 50% plus 1. I have to emphasize though that its not just tumbling skills that the girls will need to max out on........ they will need to be doing level 3 stunting and baskets as well (extended 1 leg stunts, full downs from 2 legs, and 360 twist baskets) to max out in all areas, as well as be clean with the skills they perform.
Now, that being said, there are a significantly less number of youth lv 3 teams vs. youth lv 2.......... you may go against 1-2 teams all season, so you may win for just being in the division when no one else is there at some competitions.