You can't mix Social Security retirement benefits and disability benefits. The two aren't really separate programs: disability benefits are based on the same earning formula as Social Security retirement -- you just get them earlier in life. If you're receiving disability when you reach retirement age, you start receiving retirement benefits instead. The amount doesn't change, just the way the government classifies it.
One exception to getting disability in retirement is if you quit work because of a disability, then applied for early retirement benefits. Early retirement benefits are less than what you get at full retirement. If you apply for and get disability benefits however, the Social Security Administration will give you full disability -- equal to your full retirement benefits -- until you're old enough to qualify for full retirement pay.
Boosting early retirement benefits with disability benefits isn't a slam-dunk. The Social Security Administration could decide, for instance, that your disability came after you retired, in which case you don't qualify for the benefit boost. The SSA could also rule that you aren't disabled by its standards, so you don't get any disability at all. In that case you're stuck with only the early retirement benefits. Taking early retirement also reduces how much you get at full retirement if you don't qualify for disability.
You only get Social Security disability benefits if you're totally disabled, meaning that the disability has to be expected to last at least a year -- or less, if it's expected to kill you. It also has to make it impossible for you to continue in your career or to adjust to other work. The SSA has a list of conditions such as Lou Gehrig's disease and acute leukemia that get automatic approval. Otherwise you have to prove your qualification to receive disability.
If you don't have one of the automatic qualifiers, the SSA will review the details of both your job and your disability. The reviewer will look at factors such as your ability to exert yourself physically, manipulate objects, walk around or concentrate on a task to decide if you can still do your job. She also assesses your abilities, education and work experience in deciding whether you can adapt to another job.