(ET) 8v battery in the mix?

[A-M <amcool gbta net>]

My pack is probably 7-10 years old, and still does all I need it to--or 
did until I found one battery open circuit this spring.

As I understand it, putting one new battery in with the other 5 does no 
favors to the new guy.

My local golf cart dealer will basically exchange a 'decent' old trade-in 
battery --when he gets one--for my dead one.  He's already been through 
the spring rush though, so no idea when that might be.

He does have some trade-in deep cycle 8v batteries though, which seem to 
be the same physical size.  Any chance adding one to the mix might not be 
too much, until a 6v shows up??  

I do have a couple of ICE riders I can use, if this is a bad idea. 

Thanks.

Thon