This arrangement is a transcription of Don Reno's playing. If you want to learn to pick it the way Don did, then this is the one for you. There's also some great licks to check out. Take a look at those for some new ideas to apply to this arrangement and your playing.
Try wrapping your thumb over the top of the neck to fret those 5th string notes. You may find it's really comfortable. If not, there's no rules as to what fingers should be used. If it works, it works.