In a modern day and up setting something described as a legion makes me think of some sort of modified soldiers. Super soldiers, robots, power armor, etc.
In a fantasy setting, ranger makes me think of the D&D / Lord of the Rings type with bows or dual wielding or stuff. In that context, a legion makes me think of some sort of organized soldier / army group. My mental image of Rangers are too independent for that, so I don't really see a legionnaire as an elite form of that.
And the name may well have changed its meaning over the years.
In the British Army, there are Dragoon Cavalry that don't have horses [or Dragons], Grenadiers that don't fight with grenades, Fusiliers that don't have muskets...
Almost any name can work, if there is a reason for it.
Legionnaire makes me think of the French Foreign Legion, not Rome.
According to Wikipedia [dangerous I know] "The Roman legionary (Latin: legionarius, pl. legionarii) was a professional heavy infantryman of the Roman army after the Marian reforms."