I want to apply for SSDI. If it is denied, can I still apply for SSI?
ANSWER: Yes, but if you believe you may meet the non-medical requirements (i.e. financial) for SSI right now, then you'd actually want to file a "concurrent claim" - i.e. file both claims together. If you delay filing for SSI, you will get no retro benefits, should you later prevail on that claim, for months prior to the date you later applied. As such, file is as soon as you are feasibly poor enough.
I'll add that what makes us "poor enough" is pretty strict, and depends on both income and resource limits. If you have any question on this, feel free to visit our "Affordable SS Q&A" page for a low cost consult. In addition, if you don't have a dedicated SSD attorney retained, re-think that - while you can wait until you are denied, the chances that you are going to contribute to your own denial by the quality of the input you make into your application and documents thereafter is, I believe, substantial. Since most quality SSD lawyers do not change a retainer, money is not a bar and you can get one today if you wish*
*depending on whether you have a viable claim, as determined by a reviewing attorney after thorough phone interview.