Hello guinea fowl do best on high protien - meat bird, game bird, turkey starter or food depending on age. crumbles of course for babies can go to pellets for adults. if feeding organic and before free ranging, need to concentrate on protien along with other nutrients. soy, meal worms, sunflower seeds etc. they love millet and milo some scratch grain contain millet or milo or both. if you would like to email me it is email@example.com i am not on computer every day. we are in western ky. hope you enjoy your critters.
Young Guineas (6+wks old) usually go thru a gangly teenage stage for several weeks, so it's a good idea to make sure their main diet consists of no less than 20% protein during this age range, free choice.
Cracked corn, scratch, even sweet feed will all help fatten up a Guinea. You don't want them fat tho, it's unhealthy. You want them to build muscle/meat... and protein helps with that, not corn/starches/fat.
Feeding a layer feed with 16% protein free choice is all adult Guineas really need, especially if they are free ranging also.
If they are thin there may be a worm infestation causing them to be underweight. Maybe take a sample to a vet and have them do a fecal to see if worms are the culprit.