The charts, and indeed the radio stations, weren’t “specialized” in the '50s like they are today. Until the mid-to-late-'60s, it was all classified as “pop” music, and you would hear Elvis, Frank Sinatra, Johnny Cash and The Beatles all on the same station, and listed on the same chart.
And about All 4 One and John Michael Montgomery recording the same songs: it really wasn’t a matter of one “stealing” from the other. Neither artist wrote the songs in question. As is very common in both pop and country music, the artists recorded songs written by outside professional songwriters. A songwriter will pitch his work to different record companies and different artists, and the writer is happy to sell the song to whoever is interested. If two or more artists want to record the same song, so much the better for the writer. If I recall correctly, both of these artists coincidentally happened to record “I Can Love You Like That” at the same time, neither knowing about the other artist’s recording, and their albums were released at about the same time, and both ended up having a hit with the song. The proximity of the release dates precludes one of them “stealing” from the other.
And of course, it’s usually not the artist who decides which songs are released as singles, or get played on the radio. It’s the record companies and the radio stations. So it’s quite likely that, with both albums coming out at the same time, All 4 One’s record company released the song as a single first, and then when Montgomery’s record company saw that the song was a hit they decided to release his version of the song.
I think Montgomery (though it could have been someone else - I can’t remember) recorded “Friends In Low Places” at almost the same time as Garth Brooks, also.