AL80 work fine as a pony IMO.
Some friends and I have done "monkey mount" a few times - regular single tank BP/W with a tank on our backs and one or two AL80 slung. We did this in Bonaire, taking a 88min dive to the far side of the 2nd reef, where no other divers were - what a treat!
I am over-weighted with a SS plate, a 3mm full suit, and one AL80, so i just put 2lbs on each bottle slung. Blowing them down to 500 on our first test dive didn't result in any issues and I could easily hold my safety stop.