Generally, for prototype or repair, you should use "no-clean" flux only. I think, most of the fluxes used in SMT don't produce conductive residues, but you should know for your utilized products. From this prerequisite, I don't see a particular difference between 0603 and 0402 parts in reflow soldering, apart from some size difference. I generally use 0402 decoupling capacitors at the bottom of FBGA components, particularly FPGA, cause they exactly fit the pin grid. The assembly companies are not having issues with it, even a small company, that assembled of BGA components for the first time two years ago on my request (with serious doubts in the beginning).