In the 5s bracket, 4-boxing with another player playing the 5th is very common for the higher performing teams.
Playing all five, is very different... and that will remove a lot of the more competitive teams.

The majority of the videos of 5s teams that were competitive in their season, has been 4-boxing with someone else playing the healer.

I'm not saying don't go with 'you must play all five in a 5s match' or 'all three in a 3s match'.
Just that that will greatly reduce the quality of the teams.

Then again, everyone can play their entire team.
Not everyone can get a fifth/third person who is very skilled in arena play...