Oh gosh, way to confuse me!
I stuck with the Rifts and Dark Conversion data, as first it appears that the fantasy stuff is very similar, and almost a reprint (PF was 1998, DC was 2002) of the stuff in the Dark Conversion book. So it appears that "new rules" were written for Rifts.
However, yeah, the whole lesser/greater thing is kinda unclear. I thought it made sense though. Those big super bad-arse ones are hard to summon, and only really high level warlocks can even attempt to do so, and they fail most of the time they try. However, those big ones, and the lower level warlocks, can all summon all those lesser elemental types.
Hopefully I'm right on this, otherwise.... well ugh, confusion!
So, for me, and the way I'm treating this now:
"Greater Air Elemental" = "Major Air Elemental" = Rifts Conversion Book 3 - Dark Conversions, Page 109 - Attempt to summon once/day, base percentages.
"Lesser Air Elemental" = "Minor Air Elemental" = Rifts Conversion Book 3 - Dark Conversions, Page 110 - Attempt to summon once/day if level 9+, half percentages.
"Greater Earth Elemental" = "Major Earth Elemental" = Rifts Conversion Book 3 - Dark Conversions, Page 111 - Attempt to summon once/day if level 9+, half percentages.
"Lesser Earth Elemental" = "Major or Minor Lava Monster" = Rifts Conversion Book 3 - Dark Conversions, Page 112 - Attempt to summon once/day, base percentages.
"Lesser Earth Elemental" = "Minor Earth Monster" = Rifts Conversion Book 3 - Dark Conversions, Page 113 - Attempt to summon once/day, base percentages.
"Lesser Earth Elemental" = "Mud Mound" = Rifts Conversion Book 3 - Dark Conversions, Page 113 - Attempt to summon once/day, base percentages.
"Lesser Earth Elemental" = "Earth-Tree or Plant Elemental" = Rifts Conversion Book 3 - Dark Conversions, Page 114 - Attempt to summon once/day, base percentages.
"Greater Fire Elemental" = "Major Fire Elemental" = Rifts Conversion Book 3 - Dark Conversions, Page 115 - Attempt to summon once/day if level 9+, half percentages.
"Lesser Fire Elemental" = "Minor Fire Elemental" = Rifts Conversion Book 3 - Dark Conversions, Page 116 - Attempt to summon once/day, base percentages.
"Greater Water Elemental" = "Major Water Elemental" = Rifts Conversion Book 3 - Dark Conversions, Page 117 - Attempt to summon once/day if level 9+, half percentages.
"Lesser Water Elemental" = "Minor Water Elemental" = Rifts Conversion Book 3 - Dark Conversions, Page 118 - Attempt to summon once/day, base percentages.
"Lesser Water Elemental" = "Ice Monster" = Rifts Conversion Book 3 - Dark Conversions, Page 118 - Attempt to summon once/day, base percentages.
These are all the ones that require "summoning", or a planned out event out of combat.
DURING combat, warlocks can use the following spells to summon these kinda creatures, which aren't as good as Greater/Lesser (or Major/Minor), but can sometimes be useful:
Level 4 Air - Leaf Rustler (worthless melee fighter/mule)
Level 4 Air - Phantom Footman (decent melee fighter/mule)
Level 5 Air - Phantom (good melee fighter/mule)
Level 5 Air - Phantom Mount (its a horse, yippee)
Level 5 Earth - Little Mud Mount (Tough and pretty good melee fighter, can also cast spells, very useful)
Level 8 Earth - Create Golem (decent melee fighter)
Level 4 Fire - Flame Friend (decent melee fighter/mule)
Level 6 Fire - Dancing Fires (not really a creature, but treated like one. Poor melee fighter, kinda silly)
Level 6 Water - Little Ice Monster (Tough and pretty good melee fighter, can also cast spells, very useful)
Level 6 Water - Summon Sharks/Whales (1 per level of them, quite useful, in very specific situations)
The only limit to those would be the PPE available, which could produce quite a few of the crappier units, and a couple of the better ones.
Note that "Greater Elementals" can also summon as many as 4d6 "Lesser Elementals" up to 4 times a day, making even 1 an army.
Let me know if I'm somehow wrong