My advice is either way you go, you still need to dose NPK. Plus I would change to twice a week dosing the flourish comprehensive, and you should be ok with using osmocote plus as root tabs (1 tab per square foot of tank space). The osmocote plus tabs should be fine as a substitute of the NPK, since osmocote contains nitrate, phosphate, and potassium.
Which flourish product are you using? If it's flourish comprehensive, it pretty much contains micro nutrients. SeaChem needs to change the name of it to Flourish Micro if you ask me.
So if you're using flourish comprehensive, and then Osmocote Plus root tabs or root cicles you should be fine with only using them together. Now what are you doing about co2?
15g Fluval Flex - Will be a planted shrimp tank