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
just do the full science load and get it over with. you need to show you can handle a full course load at once. thats what admissions is looking for in an applicant if you have poor grades. you can work afterwards. plus working + taking classes is not a fun time.