@jmv Do you currently have a baby, or are you planning ahead? Question mainly related to what sizes to get to try.
Having to boost them depends on baby's age and how often you want to change them! I never needed to boost the newborn ones - mostly because baby POOPS so much as a newborn that you're changing it so often that it's rare for them to out-pee the workorse, lol!
Older babies, though? I definitely had to boost them - I really didn't use them much between newborn and older toddler stage, so I don't have much experience in that age bracket, but I am confident I would need to add doublers.
Now as a 26 month old, when she's at daycare, I load her up with large or X-large GMD workhorses because they're super easy for the grandparents to use. I bolster each one with either a GMD newbie prefold (fits perfectly in the large) or a GMD Novice prefold (fit perfectly in the x-large). With this bolstering, she generally goes through only 3-5 diapers between 7am and 7pm. If she was getting changed every couple hours, then I really wouldn't need to bolster them.
Many brands of covers fit over them, for sure! The brands I've used with a lot of success are: Thirsties (both duo wraps and sized covers) Cloth-ezz wraps (unsurprisingly, they give a very comprehensive size-fit guide on their website), Nicki's Diapers covers, Kawaii Baby covers, Rumparooz one-size covers.