Swarms are a different type of monster to other creatures, but for the sake of simplicity they generally aren't treated that differently from other monsters. I'd like to share and discuss alternate rules to apply to all swarms.
  It could be best to treat swarms as immune or double resistant to sneak attack damage because even though ghosts and stuff can be sneak attacked, I still wouldn't want rogues beyond level 5 or so to be able to pull off the sweetest trick shots and skewer 10 rats on an arrow or thrown knife. I actually feel like it's because sneak attack is usable against every creature as far as I know that I feel comfortable making swarms ignore it. I don't think anyone is going to make the case that making rogues bad against swarms is too big of a nerf to rogues. Usually swarms are used at lower levels when sneak attack damage isn't such a massive portion of the damage. 
  I also usually rule swarms as taking 2x or 1.5x damage from AoE attacks, if my player is going to think "gee it's many creatures all globbed up in one spot, might be worth it to drop a thunderclap or burning hands instead of firebolt" I feel like that makes sense. It also gives rogues another option for damaging swarms if things like alchemical fire are effective against them (even though that's not technically AoE damage), it's also just fun to mix it up and use a consumable item now and again. 
  I have swarm on swarm combat ignore the physical damage resistance they have, for kind of obvious reasons. 
If anyone has any thoughts on these rules or have ideas for their own I'd love it if you'd share your thoughts here. 

I use the following rules for swarms, some similar to what you have,, taken from looking at swarms in 5e and 4e.

• Swarms are resistant to bludgeoning, piercing, and slashing damage but take more damage from area of effect attacks, no matter the damage type.
• They have the ability to swarm creatures of a larger size (than the constituent parts) and move through openings of a size equal to the constituent creature.
• An automatic damage effect vs. those enveloped by the swarm. Once the swarm is bloodied, there are no longer enough members of the swarm to continue the automatic damage.
• A melee (usually) attack which deals half damage once the swarm is bloodied.

This grants the following features to all swarms.

• Vulnerable. Area of Effect attacks.
• Damage Resistances. Bludgeoning, Piercing, and Slashing
• Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny creature. The swarm can't regain hit points or gain temporary hit points.

 An example of the attacks (taken from my swarm of crawling claws) are:

• Jabbing Fingernails. The constant jabbing of the crawling claws' fingernails deal 1d4 piercing damage at the start of an enveloped target's turn. This damage no longer applies if the swarm is bloodied.
• Claw. Melee weapon attack. +3 to hit, reach 0 ft., one target in the swarms space. Hit. 4d4 bludgeoning or slashing damage or 2d4 if the swarm is bloodied.