OK, lets see if I can explain all this and make it understandable. I am not the best at this so bear with me.
General rules of breeding. (please remember there are exceptions to everything)
You should use a buck that is smaller than your does, unless you have a heavily bred doe that can handle giving birth to a larger kit.
For small breed rabbits: You should breed for your first litter after 5 months old and before a year. It is extremely difficult for a doe to give birth after a year. (Most breeds fall into this catagory)
For large breed: You shouldn't breed your doe before 6 months and preferable have them give birth before 14 months old. These are your meat breeds, flemish etc.
If you doe has a difficult birth and looses her kit's but did give birth, you want to rebreed her sooner than later, almost the sooner the better because she will already be stretch and loose to give birth again.
As for kindling does, you don't want to interfere to much unless you have to, that can stress them out more and cause birthing problems from them being to hyper and not focusing on their jobs.
As for a single kit, if she is nursing it and it has a nice nest, 60 should be ok. Sometimes, does won't mess with a single kit and sometimes the does know better then us about what babies should survive. I have found that saving kits for later breeding is NOT a good idea, generally the kit will have something wrong with it long term. Also, mothering traits are passed on from your doe, bad mom doe, then her daughters generally will be even worse.
Hope this helps you guys. And remember there are ALWAYS exceptions. I had a Flemish doe give birth just fine at 2 yrs old. Her sister died in birth.