The mana/damage ratio is all well and good when you're looking at 1 toon, or even 5. But at 26 toons I'd be more inclined to look at it as 1 cast. There isn't a none-elite mob in the game that at same level will withstand 25 lightning bolts...

Example:

Generic bear 670 Hp at lvl 21
Level 20 Lightning Bolt does 83-95 damage
Most damaging shock at 20 is Frost Shock 89-95 damage (highest damage as most recently obtained)

I'd argue that the team tagging mobs should be casting a shock, while the remaining 5 teams begin lightning bolt, either way lets assume that all the spells do around an average of 70 damage taking into account some partial and some full resists.

25 (you have 1 pali iirc?) x 70 = 1750 damage - this is also known as dead bear...

So it all comes down to how fast you can target with the main, assist on the other 25 characters, cast a 1.5sec spell and repeat (add time to loot if you're looting)

Anywhere between 3 and 5 seconds sounds about right.

At 400 mana each character and a spell cost of 115 mana you'll get less than 4 casts off.

Lets assume that you allow 3.5 to 4 seconds so that you get mana regen between kills (it kicks in even while casting, until the cast ends iirc)

Later you'll have talents that can massively increase your up time, and blessing of wisdom will also help alot here.

Any items that give your shamis +intellect/spirit/mana regen are great at any level while grinding or questing.

If you are pausing now n then to regen mana, it's not going to make a huge dent in your kill/hour ratio as you're casting 1 spell, brief pause, next mob.

You're never* going to be killing faster because you simply can't kill faster, it won't be down to mana

*If you're toons can take hits and you find a decent area for it, you can run around and grab loads of mobs, throwing down the Paladin AoE spell consecration to keep them "tagged" (to stop them losing interest and resetting/evading) and then drop 25 Fire Nova Totems (for this you'll need the guys NOT getting XP to drop em a half second late to make sure the right group gets XP, plus the pali's damage will tag most of em before that, but if you get a method down you can AoE fairly well, upping you're kill/hour.

To be honest there are very very few areas that can support that kind of kill/hour ratio, if any once you get a bit higher.

Consider this: a maximum of 1 group at a time is getting XP as you kill, if they need 141 mobs to level (20-21) OR say 7 quests, why not make those 141 mobs you're killing anyway count for 7 quests, and get TWO levels for the price of ONE? Better yet, kill say 80 mobs for 3 quests and get 1.5 levels 61 mobs faster. You're killing them anyway!

Plan it like this: Arrive in "Town A" set ALL youre hearthstones to the local inn near the quest hub, grab all the quests, head out into an area and do quests in that area one team at a time, then move along to do another couple, all the while killing as you go. Once all 26 toons have a bunch of quests done, hearthstone HALF of them, hand in all the quests, then RAF Summon them back, then hearthstone the other half and repeat with them. You've now handed in a bunch of quests (which is made easy with Jamba/other) and have a buttload more XP to show for shit you were killing anyway, you have some items you can use on ALL toons which will increase your mana pool/damage/regen/whatever and some cash, or at worst they can't use the items and they sell them... still cash either way!

All it comes down to is a little planning and forward thought, which is something against you as you haven't played the game before, I'd honestly go buy Joana's Guide, and just drop any quest that takes you out of your way or sounds annoying. Just go 1 zone at a time and utilize the RAF summons creatively as suggested.

I honestly think combining questing with the grinding will reduce the levelling time by a massive ammount, at least 50%, and more at higher levels, as the XP/mob doesn't scale like quests do, and neither will your kills/hour purely due to game mechanics...