I think even with a squad of 7 you would end up with alot of sub standard teams turning up, No GP riders will do friday Saturday, Sunday meetings. Extra league riders would be missing much of the time.
I think to merge the leagues you would have to lower the standard considerably, say good bye to the riders who wish to ride polish League and replace them with doubling up national league riders (you would need 7 additional riders).
Not popular opinion on here but I would try to convince a team to change race night, move up to make it a 6 team league and fufil the tv contract, Redcar? Glasgow?