Quote from Kyreeeeeeeeee >>

" The new form can be any beast whose challenge rating is equal to or less than the target's (or the target's level, if it doesn't have a challenge rating). "

Definitely dislike this. The spell shouldn't be limited by target's level/cr, but instead the caster's. The limit should be based off of the caster's power. That being said, I guess they wouldn't want to have a wizard cast this on their familiar and then have a t-rex stomping all over the place. 

You answered your own grievance there, basing it on the caster's level would be broken from a mechanics standpoint. From a narrative standpoint, consider that the magic can only shape what is already there... the inherent power that a creature possesses (mechanically expressed by levels or CR) is like a clay that the spell can mold and shape into a new form. No matter how skilled an artist is, they cannot turn a pot's worth of clay into a man-sized statue, nor a man-sized statue into a building. 

So, technically, a swarm of cranium rats is a beast... Confusion/dominate monster once per day, and a bunch of at will spells :D

As a sorcerer, if I twin this do the two targets have to be polymorphed similarly? I might want to change an ally to a T. Rex and an enemy to a mouse. 

 The rules don't really state either way, so its a DM call if the variables have to be the same when twinned. I am not aware of any sage advice on it.

As a general rule, I'd say since it doesn't expressly say the spells have to be cast the same way then that limitation doesn't exist. So I'd let you polymorph the friend into the Trex the enemy into the mouse.

Could a player polymorph a living creature into an undead?

step one on how to piss off your dm, and everyone else around the table with your overthought bullshit: step one, get a bag of holding and be a high-level wizard, step two get the glyph of warding spell and glyph thirty copper coins with this spell(we're working with the assumption that just like any other spell you use with glyph of warding, it won't be cast on the glyphed item) and then put the coins in the bag of holding, then when a truly awful NPC, or a bbeg, or just an annoying ******* NPC if your the murder hobo-y kind, comes around spill the coins out and then cast the spell on them with each and every coin, and if your dm is okay with that horrifying bullshit, turn the NPC into a horrible amalgamation of weak creatures, basically turning the NPC into a sick hybrid of every creature that you can think of off the top of your head, basically make them a more sickly Frankenstein's monster,and then in the brief moments between life and death, they will understand their error, and then their organs will just give up, pretty much overworking incompatible body parts, until it dies, probably just a few seconds later. 

My character is a level 5 Bard, it says you can have polymorph at level 4, but I'm not seeing it as something I can get or is it one of those things I have to pay for?

If I polymorph something like a dragon into a spider and if I then petrified the spider, would it remain a spider till it's not petrified anymore? Would it turn into a petrified dragon after an hour?

Also if I were to crush this petrified and polymorphed  spider into a million pieces would that instantly kill the dragon or would it turn back into the dragon it was before it was polymorphed. 

just one question, do you lose access to class features that arent magic? like do you lose stunning strike if your a monk who gets polymorphed? 

Could a Moon Druid who has been polymorphed by an enemy simply use a bonus action Wild Shape to effectively end the Polymorph?

as a druid can i cast it on someone and, the creature i choose does it count as a seen creature for wildshape

Is this the right spell to use if you want to do minor modifications...say like 'remove ears' or 'fuse eyelids'?

No, you can't. You immediately drop the concentration even before you transformed.

really, i hold almost opposite

YOU LOOSE ALL CLASS FEATURES!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

"or take any other action that requires hands or speech."

me who turns into a monkey :l

If you have a mimic that polymorphs into an object, is it counted as a creature or an object in its polymorph form?

What happens if you polymorph something into a 1HP spider and then feed it to your familiar?

If I am a Human, Circle of the Moon Druid (with shapechanger special ability) can I cast it on myself (because it could go both ways...I am a shapechanger or is that just a special ability so I am still a human)? My DM is leaning towards I cannot cast it on myself.

No. It means beasts.