Will keflex 250 help with acne? Is this dose too low to make a difference?

Keflex/acne. No. Antibiotics need to be targeted against specific bacteria which caused a problem. In this case acne is not one which can be helped with keflex.
Maybe. Keflex might lessen your acne, but that's not what it is for. The best current antibiotics for acne are Doxycycline and minocycline, with a few others used less commonly - but antibiotics are not that effective all by themselves. Let your dermatologist design a comprehensive acne treatment for you, which will probably include an antibiotic and appropriate topical therapies.