1/32 was actually the original scale for large scale helos. One of the first was the ICM Bell Jet Ranger in the mid-1960s; the actual date has been lost to history.
Revell started with 1/32 helos as well. Their first ones were the UH-1D in 1967, OH-6A in 1970, and AH-1G about 1970 as well.
The 1/32 military helo trend was continued by Revell when they bought the ICM Jet Ranger and repackaged it as an OH-58A in 1986.
Revell has continued with 1/32 helo models and has a lot of them available. They are mostly of European design though.
The very first 1/35th helicopter model was the Heller SA321 from 1975. It was a one-off in 1/35 at the time though. It was actually a pretty good model; well detailed and nicely molded.
The 1/35 modern helicopter standard didn’t start until Seminar made a 1/35 UH-1B (kit #8000) in 1990.
MRC/Academy then released their USMC AH-1W (kit #BA-100) in 1992.
After these two kits were very successful, others jumped on the band wagon to include Dragon, Trumpeter, Panda, Kitty Hawk, etc…
Now 1/35 is pretty much the standard scale for military helicopters so they go well with all the 1/35 ground vehicles, figures, and accessories.
The difference between the two scales is relatively small, sort of. 1/32 is 9.143% larger than 1/35. You can use the two scales together as long as constant-sized items such as weapons, canteens, ammo pouches, etc. are all the same scale. I have used 1/35 pilots, soldiers, and gear with 1/32 helicopters in the past. They look fine together. If you go the other way though, 1/32 figures with 1/35 helicopters or vehicles, it doesn’t work as well since the figures are too big, almost 10% larger.