Hi flowerfairy, I have 2 guinea pigs - but looking at the amount of poop, you'd think there were at least a dozen!
In their cage I use a single layer of newspaper (just to make it easier to remove the Megazorb in the weekly deep clean), then a layer of Megazorb, then another layer of newspaper (to make it easier to refresh the hay daily) with hay on top. Every day I remove the top layer of newspaper/hay/poop and replace, putting the used stuff in the compost bin. Once a week I empty the lot - and it all goes in the compost bin as it's all plant-based (Megazorb and newspaper are wood pulp). I add lawn clippings, other garden waste and plant-based kitchen waste.
I'm not actually sure how long it takes to compost down... I have two bins, so I fill one, and then start filling the other. By the time I'm nearly ready to start a new bin again, the first bin has composted down so I riddle it with a garden sieve, adding any larger / non-composted bits to the current compost bin, and then start again. I'd guess at about 6-9 months, but I know you can encourage it to break down more quickly if you're prepared to turn the compost to aerate it. (You can get "compost tumblers" which claim to make compost in 6 weeks, but I've not tried them).
I would imagine with your 6 rabbits and 6 guineas, you'd soon have a fantastic supply of compost! If your bedding is all plant-based (hay, hemp, newspaper) you'd be fine to compost the lot.
I will try to take some photos... bear with me - it might not be until the weekend!
I hope this helps... if anyone has any other ideas, please let me know!