For absorption needle length does not matter. Infact even number of needles doesn't matter.
I used 520 needle dermaroller when I had a buzz cut but now my hair is relatively longer and I started pulling hair so I moved to 192 needle dermaroller.
The length I'm using for improved absorption of minoxidil is 1.5 mm and I do not push hard enough to bleed at all. I would say that if 10 is the pressure required to completely embed the dermaroller deep into my scalp, I'm using 4/10 to 5/10 amount of force. Just enough to get reddish sort of scalp and a little tenderness next morning.
Now last time I added proper deep dermarolling with 1.5mm and god it hurt like hell and I just couldn't do it. I applied minoxidil right after and it stung but I will continue to do so because on tressless reddit sub and here I've seen a few remarkable results on people who used this technique and they did not suffer side effects. I will try 1 mm dermaroller this time.
I will apply minoxidil immediately afterwards if I'm using dermaroller for absorption only.