The problem with six in the circuit is that unless they are flying at the same speed it won't work.
Either the faster one will end up doing a go-around, sometimes several times, before getting down, or he will have to extend the circuit downwind to create more distance before himself, and if he does that then it's pretty likely that some smartar*e behind him will cut him up on the inside.
That's what it is like at my two favourite examples of a free for all: Stapleford and Wellesbourne. The first one is particularly terrible and I have wondered whether some of the instructors even have a PPL.
I don't see that oval circuits will make a difference at these free for all locations, because few people bother to stick to the rule that you have to follow the previous traffic. They won't work, just like the rectangular ones don't work very well either. Would be OK on weekdays though.