Are the figures from the BTAS and TNBA lines in scale with JLU? I know unusually large or small figures would probably look fine (e.g., Clayface, TNBA Robin), but I'm wondering about other characters like Mr. Freeze and Creeper.

Most are slightly too big, but many fit just fine (such as your examples). Creeper IMO is one of the most obviously out of place/scale from those lines when mixed with JLU.

Creeper and Riddler are the main problem. they are amazing but a bit too tall. Nightwing is as tall as martian manhunter, mmmm
Robin is fine he is as tall as supergirl and the titans even a bit taller
Mr Freeze its perfect! one of the best animated figures Ive ever seen in my life.
Forget about the Joker and Twoface.

Rah and Thalia 4 pack are a must, Thalia es amazing and Rah with cape too

Freeze is fine. I don't mind Insect Body Freeze (or the original one) being large at all. Creeper is too tall. I've trimmed one down by taking a quarter-inch out of his leg at the bootline, and that brings him much more into scale.

Female characters work pretty well, like Talia and Poison Ivy. Harley is a bit too tall, but not bad in my opinion. I'm also okay with some of the taller villains, like Ra's al Ghul. I don't mind him being imposing. And for that reason, Scarecrow works for me, although there's not a TNBA sculpt of him.

Obviously, there's no problem with the big boys, like Clayface, Man-Bat or Bane.

I wouldn't be surprised to see these figures resurface under the new brand.

I have Ra's, Talia, Scarecrow, Poison Ivy and Mr. Freeze in my JLU display and they look great.

well, for clayface, you could chalk it up to him being beefed for a fight.

I think Riddler fits in good...I just think of him being tall and lanky. I sliced a small bit of Creepers and Freezes legs to make them a tad shorter