Subcutaneous fluid needle size

The vet gave me 18g needles for my cat’s new prescription of subcutaneous fluids 3x weekly but I’m looking to change to 20 or 21g due to the pain of insertion. I know it takes a bit longer but I can handle that! I’m going to buy the Terumo brand because I’ve heard they’re the smoothest to insert. I’m trying to figure out if I should buy the normal needles, or the butterfly needles? Does anyone have any personal experience on which is easiest and most painless for kitty? And can the butterfly needles attach to the tubing for lactated ringers? Thanks!