Movement and actions in and around other creature's spaces

[stewstew5]

What are the rules for occupying spaces of other creatures of different sizes? I know you can freely move through and occupy the spaces of a creature at least two sizes larger than you, but what about multiple smaller creatures in a single space? are there even rules regarding that?

[Laserlight]

PHB: MOVING AROUND OTHER CREATURES

You can move through a nonhostile creature's space. In contrast, you can move through a hostile creature's space only if the creature is at least two sizes larger or smaller than you. Remember that another creature's space is difficult terrain for you.

Whether a creature is a friend or an enemy, you can't willingly end your move in its space

Note however that a Tiny creature's space is 2.5 x 2.5ft, so you can fit four of them in the same 5x5 floorspace as a Small or Medium creature.

[Laserlight]

BTW, this "5x5" thing is purely D&D; if you wanted to change your square size to 3x3, that would be reasonable. 2.5 x 2x5 for humans would be getting a bit tight; I'd say you need to be familiar with Martial weapons to know how to operate in a shieldwall.
Movies almost invariably show melee troops running too fast and waaaaay too spread out.

[RSP]

What are the rules for occupying spaces of other creatures of different sizes? I know you can freely move through and occupy the spaces of a creature at least two sizes larger than you, but what about multiple smaller creatures in a single space? are there even rules regarding that?

Laser cites the relevant rules. The main thing is you can’t occupy another creatures space though sometimes you can move through it.