Part-time science courses and full-time job should be ok. Just make sure your taking more than 1 class each semester.
Also, I believe the # of credits you take is dependent on your individual situation. I took ~23 credits of upper level biology courses while working full-time (7-8 credits/semester)... but I also showed a strong upward trend from my sophomore year of undergrad. If you have had poor grades your final year of undergrad, I would suspect that ~30+ credits would be needed to demonstrate academic ability