The Pathfinder campaign I am a part of just hit level 7, meaning that my cleric can now access Lvl 4 Spells. Among these is the spell Giant Vermin, which turns a variable number of tiny centipedes, spiders or scorpions into their "Giant" or medium sized counterparts. My plan, for the foreseeable future, is to use the spell Summon Minor Monster to get such vermin on the battlefield and buff them into their stronger forms. The only slight issue i have is with what happens during the instant my summons grow. Since the summons are initially tiny, they can exist in the same square as a medium+ creature, but if they are in the same square as an other creature, possibly on them, when hit with the Giant Vermin spell, would the summon simply be shunted away from the other creature or would something more complicated happen?
A GM may rule that the giant vermin spell just fails…
Magic on Spell Failure says, "If you ever try to cast a spell in conditions where the characteristics of the spell cannot be made to conform, the casting fails and the spell is wasted." This Pathfinder rule comes directly from the SRD for D&D 3.5e, and that game's Player's Handbook offers a little more detail:
If you ever try to cast a spell in conditions where the characteristics of the spell (range, area, or the like) cannot be made to conform, the casting fails and the spell is wasted. For example, if you cast charm person on a dog, the spell fails because a dog is the wrong sort of target for the spell. (171)
(The Rules Compendium for D&D 3.5e alters this slightly to the following: "If you cast a spell in conditions when the characteristics and limitations of the spell can’t be made to conform, the casting fails and the spell is wasted" (133; emphasis mine). As it was already pretty vague, I'm not sure why the Compendium makes this subtle change, but someone must've thought it important.) Anyway, the GM may rule that because of the other creature's interference, there's simply not enough space available for the giant vermin spell to take effect normally, and the giant vermin spell simply fails and is wasted.
…But this GM wouldn't
This GM would, instead, have the creature that's been the subject of the spell giant vermin moved—automatically, off-turn, and not provoking attacks of opportunity—to the nearest space that can accommodate the creature's newly-bigger form. Ruling that the spell fails is pitiless and not a whole lot of fun.
However, the giant vermin spell will fail if employed as described
The spell summon minor monsters allows the caster to "summon 1d3 Tiny or smaller animals, such as bats, lizards, monkeys, rats, ravens, toads, or weasels" (link and emphasis mine). The spell giant vermin allows the caster to "turn a number of normal-sized centipedes, scorpions, or spiders into their giant counterparts"—that is, real-world normal-sized centipedes, scorpions, and spiders become CR ½ giant centipedes, about CR 3 giant scorpions, or CR 1 giant spiders, respectively, which are all, unfortunately, not animals but, instead, vermin.
In other words, the animals summoned by the spell summon minor monsters can't be the subjects of the spell giant vermin because the spell giant vermin only targets… um… vermin.
Which is probably okay. I mean, the strategy you're considering has your level 7 cleric take a 1-round action (not a full-round action) to cast the spell summon minor monsters to bring forth 1d3 Tiny animals then, next turn, take a standard action to cast on the resultant creatures the spell giant vermin to create, in the end, for originally 7 but now 6 rounds, but 1 about CR 3 giant scorpion. The spell summon monster IV can do that by itself and better creatures than a giant scorpion can be picked besides.
The rules are silent on exactly what happens when a creature increases their space, but we can extrapolate a useful ruling.
Moving an in-place creature would be too powerful a rider to add onto these effects. Otherwise, you could use enlarge person, giant vermin, et al, as irresistible battlefield control. That's a little overpowered for low-level spells.
The next option is to have the creature being enlarged move somewhere adjacent to their current square, either randomly or by choice of the caster. In my game, I specify that it has to be somewhere safe for the creature. No using the spell offensively to drop the creature off cliffs, into lava, etc..
If the creature has no legal space to stand adjacent to its initial square(s), it stays in place and, along with the square(s) current occupant(s), is considered squeezing. You can also require these creatures to move at its first opportunity if you'd like.
This is how I've handled these situations in my games, and it works well.
I am not aware of rules that directly tell what happens when a creature grows, but the growth is restricted by other creatures. There are rules for accidentally ending movement where one should not, and applying them would not be too far-fetched:
Accidentally Ending Movement in an Illegal Space: Sometimes a character ends its movement while moving through a space where it's not allowed to stop. When that happens, put your miniature in the last legal position you occupied, or the closest legal position, if there's a legal position that's closer.
In this case the last legal position does not exist, but the closest legal position is a reasonable concept.