5fr here and I work in a nicu and peds cicu. In the nicu sometimes we would get umbilical lines for or micro preemies, but really, 5fr/6fr worked well on most everyone, and bigger babies (>5kg) often needed 8fr.
And being that I work in two units where fever's are VERY big no-no's, I have gotten quite comfortable cathing babies, both girls and boys. And as I learned on babies I really don't have much to compare it too, boys are usually pretty easy but sometimes you can think you're in and you're really half way up the foreskin (if uncircumcised) and they won't appreciate that
Girls can be tricky as well, for both I sometimes find that after scrubbing the area with betadine, it helps to use a sterile gauze to hold the labia/foreskin back to make visualization easier since it can be slippery and difficult after the betadine.
Sweet-eze and a good swaddle never hurt too!