I don't really know d&d well, but I just looked up the spell and I think it says you can transform "any number of willing creatures". So the DM could make an argument about whether insects had the intelligence to even qualify as being able to be "willing" for that.
On the other hand, I am a big fan of Operation Dumbo Drop Multistrike
Depends on the edition. If Epic Magic is in play, 9th level spells are just the beginning. However, it gets stupidly easy to kill yourself by casting something that you really shouldn't have tried to cast.
I feel like there might be an issue where the volume of each individual elephant is so much greater than each individual fly that you won't just be pachybombarding one BBEG, but the entire area, including where the players are standing.
Honestly now that you mention it, yeah, a few hundred elephants growing in the same spot almost instantly would accelerate the outer ones to a very high speed. It'd probably end up being more like a wet, chunky mini-nuke.
For the "do the locusts consent?" question, I'm a fan of an oracle die.
I've been paying Cyberpunk RED recently, so I get the player to roll a d10 under their current LUCK stat. If they roll under, then they're in luck and they get what they want.
For funsies, I'd say that succeeding indicates the locusts are a gestalt entity that is down for anything. In the context of the campaign, the party will encounter a gestalt locust swarm that wants something from the party.
One of my favorite bits about Shadowrun in particular is that directly alongside this futuristic world of cyberdecks and prosthetics and automatic weapons, are fully fledged hauntings by things like bug ghosts and big fuck-off actual dragons (one of which owns a corporation) and other mystical crazy shit.
I've played a bunch of Cyberpunk 2077 but I haven't gotten my hands on RED, so I don't know if it has something comparable. But it sounds like you're describing an Invae nest. Notably, bug totems are the only sort of spirit in Shadowrun that requires a sacrificial host to manifest, and bug shamans will capture people in order to infest them with bug ghosts which will gestate like xenomorph babies.
The hivemind behind these Invae are pretty much a perfect slot-in for your gestalt locust swarm if you wanted, and they're only interested in one thing: breeding grounds. Would be an interesting moral choice for your party. It's a powerful entity and it can do or offer many things but its price is always going to be several corpses. Not necessarily your own, or corpses of innocents, it doesn't care about that, but it wants live bodies that will then suffer greatly and then die. And then after they die they will release many little bug ghosts, which are a nuisance at best and highly deadly at worst. The bugs ate Chicago back in 2055.
Shadowrun is great. I've never looked closely at the rules, but I've always enjoyed the lore and the setting. My players don't want to mix magic and technology, so SR is off the table.
Part of me wants to trigger the Awakening as part of our RED campaign, but then I'd have to port the rules for magic, and I'd be a jerk for making my players play Shadowrun.
It's nowhere near as powerful as it was in 3rd edition, but True Strike and Harm was a favorite tactic of my wizard and my girlfriend's cleric. Pretty much guaranteed to hit with a +20 to a touch attack. If the touch attack succeeded, target now has 1d4 HP. At that point our bard would hit them with a magic missile or acid arrow.
So first off, the insect plague isn't a creature at all, it's just a magical effect that appears like a bunch of locusts. It has no hit points or AC or Saving throws or any other game statistics.
When spells do summon a creature of some sort, they list a creature type, a maximum CR (or how many of each CR) you can summon, how to determine their initiative, their disposition towards you and your allies, what happens when they get to 0 Hp and what happens to them when the spell ends.
Animal shapes needs a few things to work that we don't have here.
First off, the spells says to choose any number of willing creatures to transform, but we don't have creatures, we have a magical effect. How could you say a magical effect that has no intelligence, will, thoughts, and lacks the ability to communicate in any way is "willing"?
Animal Shapes states the effect last the duration for each creature or until it drops to 0 HP. After that, it states that the creatures assumes the HP of the transformation, and then returns to its original HP when it's transformation drops to 0 and any excess damage applies to the original form, but the insect plague doesn't have any HP.
In between the information about HP, it states that it's game statistics are replaced by the creature choosen, other than alignment and it's mental stats. What game statistics? Insect plague doesn't have any. What happens when one of the (Large or smaller creatures) is in the air and has no mental stats? It would either totally unable to act, or dead depending on how you interpret ability scores being zero. If it's dead right after it gains it new form, it reverts to its original form, so again this just doesn't work.
The worst part about this combo, is that you could probably do this, just not using insect plague. A couple of the conjure spells summon intelligent or even sentient creatures who could at least be interpreted as being willing. Find one that can fly, send them up in the air, and ready shape animals to cast then they are 30ft away. Note that that doesn't mean 30ft up, you have to account for horizontal distance as well.