Yes a boy can wear girl pull ups and they will work just fine.
However the boy may not like the prints of the girl pull ups and may not want to wear it.
But if all you can get is the girls pull ups for your boy then they can wear it if they will.
The boys pull ups though work much better for the boys than the girls pull ups do but they will work if that's all you can get at the time.
Yes you can reuse a dry pull up.
However eventually the dry pull up can get dirty through sweat etc and then after a few days use of the pull up being dry they should still be thrown away.
The pull up will also clump up and be less comfortable for the child to wear as well.
If the child can keep a pull up dry longer than a few days then they really no longer need a pull up and can switch to regular underwear.
The pull ups are less absorbent than diapers.
The Pull Ups are basically just a less absorbent diaper that you pull up and down instead of taping them on.
The pull ups are not meant to actually hold much pee from the child but instead they are meant to catch and absorb some pee just in case the child does not make it to the toilet.
When a child is wearing pull ups they should be reminded to go to the toilet to pee and eventually the child will get the hang of going pee on their own.
Once the child can keep the pull up dry during the day then they can switch over to regular underwear.
Or you can just use underwear and then clean the underwear by washing them.
Or just keep diapers on the child which some people do and then they take them off the child when they need to pee.
However the pull ups will make it easier for the child to use the toilet themselves because they can easily pull up the pull up diaper and then remove it as well when they need to use the toilet.