Answer:
The best dogs for breeding or just having a litter are smaller dogs for a start. One because smaller dogs mutare earlier and therefor are more safe when having pups. Terriers are best out of all small dogs, dogs with flat noses dont cope too well in births, they struggle because they will become out of breath easily and find it harder to breath, then they struggle to clean up or remove the membrain from the pup, of corse these dogs can have babies but you may want to make sure you have everything you need in case you need to help or theres an emergancy, do your homework! Small dogs such as Chihuahua's and Papilons are usally fine in whelping but may also require some help in some cases, make sure that the mate of the Chihuahua/small dog is the same breed or smaller then then her otherwise it is common for the puppies to be too big and become trapped in the birth cannal which can lead to the mother dieing or futher promlems in the pup later on in his life or the mother will simply die during birth. Terriers such as Jack Russell Terriers, Border Terriers and spaniels are very adapted to coping well in births and there are usally no complications, still, do your homework, get everything you need ready which is more then you might think and always be ready to help your dog in a birth no matter what the breed. The minimum age for a SMALL dog to whelp(have pups) is 2 years(this is the safe age, although some small dogs get away with having puppies at 1-1 1/2 years of age) and 3 for bigger dogs because they muture later, some bigger dogs still not having their first season till 2 years. (dont class your dog as a 'small' dog if it is any bigger then a springer spaniel) the oldest any breed of dog can have pups is about 6 years altough many dogs can manage to push it to 7 (I no a clumber spaniel to gave birth to 5 healthy pups at 7 years of age) It is best to wait till the dog has had at least 2 seasons/heats beofore breeding her. Leave a 5 month gap (minimum) to make sure your dog is in execlent condition before breeding ie: Not overweight, hansnt got flees(deflee and deworm halfway through dams pregnacy) up to date with her shots (ask your vetarian when to get your dogs shots done, different vets say different things, some will say during pregnancy, some will say before)
Hope this helps:)