Well some of the players who've played against Marshall at his fastest claim that he was around the same speed as 'superfast guys' like Lillee, Bishop, Roberts, etc.
True, he might've been short of the '99mph' list of bowlers like Holding,Thommo,Akhtar,Lee,Waqar, etc. but he was easily over 90mph on a consistent basis from early 80s to late 80s.
Based on what the batsmen say, Marshall's air speed was likely in the low 90s zone but he was almost as fast as anybody after pitching(due to the skid factor, like you mentioned). The 'slowest' amongst the West Indies pace quartet was Garner, who was apparently usually in the mid-high 80s zone but got such disconcerting bounce and had such a pin-point accuracy that he was deadly. Although it must be mentioned that Garner had a series in Australia in the early 80s when he was bowling seriously fast. Alan Border i am sure, remembers.
As per the difference between medium fast and fast medium, i am not 100% sure but i think the former means someone who usually bowls at around 85mph zone but has a spell every now and then in the high 80s/low 90s zone, like Kapil while the latter is a guy who usually bowls in the low 90s but has a spell every now and then in the mid 80s zone...like Gillespie.