To the best of my knowledge the physical games tags “Singleplayer”, “solo-rpg” and “solo” are used synonymous. Merging them into the “Singleplayer” tag seems like the most transparent choice. (If you need a reasoning for that: In case a Han Solo tag ever comes around. ;) )
Same but more obvious with the physical games tags “2-player” and “two-player”. “2-player” would seem more logical to me (because numbers are short and international), but it is used less commonly - so what do I know. ;)