Insulin needles for drawing and injecting subq?

What needles should I purchase for subq injections of BPC-157/sema? I'm seeing that some people get ones with detachable needles, and then use a larger/separate needle for drawing the medicine in, then using a smaller gauge to inject. Others use insulin needles presumably for both?

is a 4mm needle too small to draw from the vial?

I also see CVS sells pen tip needles, should I find an injectable pen that corresponds with those? Or is that not best practice?

I just want the safest and most painless option, I don't care what it costs. Is that 29-31g and 4 or 8mm insulin needles? Also is 8 too long?