Kodi - it depends what e-cigarette you get. Some of the spiffy flashy sites, mall kiosks, cigar shops might sell you something that requires their custom foil-ended cartos (the idea is to lock you into purchasing everything from them). Or some might have cartos with hard glued-on ends or other attempts to prevent refilling.
But if you get a model with refillable cartos or carts. then yes, you can use all of the e-juices from wherever you want. I recommend you read/watch the contents of the sticky threads at the top of the New members forum, particularly the videos and the Library thread's multipart guide. Those will introduce you to popular models. The only ones you might have trouble refilling in their stock form are 2-piece models, some of which might be sold with cartos that with hard ends, but for popular carto models you can get different blank cartos elsewhere. (btw cart holds the juice of a 2-piece model, carto is cart and atty in one piece)