you cannot put new stuffing in cartos
cartos are cartomizers, screw-on disposable combined cart+atty in one enclosed piece, with wicks and wires running through the stuffing if it is a stuffing type of carto
carts are cartridges and are made of just plastic and stuffing push onto or into an atomizer. For standard 510 carts try the precut blue foam stuffing at strictlyejuice.com
Everythng Riva and Ego should be compatible except maybe the cone-shaped covers
In 510 attys and cartomizers, you can use anything standard resistance or the hotter low resistance.