Calling them non-combative and birds like Rhode Island Reds and Marans as combative is a pretty opinionated statement, I'd say.
The factual differences are that Houdans are a 5 toed bird who only comes in two recognized colors, mottled and white. They're larger, heavier, (they were made to be dual purpose) and normally by standard of the breed, should look like a brick with a crest and beard. Polish on the other hand are smaller and more "ornamental," they have the normal 4 toes on each foot, a more traditional slightly lean looking body, and come in a wider choice of recognized colors, however unlike Houdans there are no mottled Polish.
Because Houdans are a rare breed, and Houdans who look like what the breed standard and what their purpose of creation was for are exceedingly rare, honestly the only difference these days is that Houdans come in mottled and have 5 toes. Otherwise their body type in most cases looks the same. They often weigh the same if not for about 1/2 to 1 lb difference, they both usually have the same body type and tail set, they're both often bearded or non-bearded (even though Houdans should never be clean-faced unlike Polish) and have small, medium, or large crests and/or combs.
In a nutshell, these days, the only difference you'll see in most cases is coloration and that extra 5th toe.If you want to get either Polish or Houdans for a backyard flock you're honestly not going to notice a difference. Hatcheries breed them both with the same idea in mind, and you'll get extremely similar birds. I don't see any behavioral or hardiness difference between my Houdans and Polish other than the fact that the line of Polish I've been breeding are remarkably courageous, tough, and curious birds. My Houdans act about the same as the Polish I kept for pets but didn't breed. There's not much to say on their temperament beyond that, because everyone's experiences differ on strain, environment, and climate.
Don't feel offended about the combative vs non-combative breeds. It's not an opinion but an experience gone through personally so be tolerant for sharing my lesson learned the hard way - something I hope will help someone else possibly experiencing a similar circumstance. I always thought a chicken was a chicken was a chicken. My folks only had Babcock Leghorns in quantity and before that a few RIRs & BRs so my experience had always been with assertive breeds until later when I started building my own little flock and found a significant difference in breed temperaments. I had discovered that the crested, bearded, muffed, feather-footed breeds turned out generally more docile in temperament than the popular heritage or heavier dual-purpose breeds. As pullets the breeds seemed compatible together until they reached about 18 months to 2 years of age and at this maturity the larger, heavier breeds bullied the smaller or gentler breeds just because they could get away with it - the gentle breeds either couldn't or wouldn't self-defend themselves. We re-homed our nice Leghorns and Marans because they were exceptionally aggressive towards our Silkies and Ameraucana. I had a couple Silkies I thought were moulting (their heads were going bald!) but then discovered the Marans cozying up to them at roost to pick off all their fluffy feathers! During the day the 7-lb Marans would jump and claw attack the 2-lb Silkies - not an equal match. I know flock politics is a chicken thing but when there are escalating attacks with potential injury I don't tolerate it. Leghorns are wonderful birds for so many reasons but the Mediterranean breed class are not shrinking violets and will take advantage of bullying if not put in their place by an equal defender. Ameraucanas and EEs would rather flee than fight so the Leghorns had a field day chasing them to pull out their beards/muffs. The Leghorns and Marans were an equal match to each other so instead shifted their focus to chase the gentler breeds. Marans are aloof and stand-offish to humans which makes them seem calm but they will suddenly jump on and claw-attack viciously a gentler breed. My friend and I have experienced this with our Marans and will not be replacing them in any variety if there are gentler or smaller breeds in the flock. I love nearly all breeds of chickens but found out the hard way that it is wiser to keep the breed sizes and temperaments similar to avoid serious flock conflicts - especially in a small suburban yard like ours.
I have to say my experience matches that of Sylvester. I have two coops now and alternate who gets to free-range. One coop houses my LF/combative breeds: two EEs, a barnvelder, a RIR mix and a Cuckoo Marans. The other houses my bantams/non-combative breeds: Cochins, d'uccles, Polish, OEGBs, and silkies.
While they were together, and if I ever free-range them together, the combative breeds pluck my little ones. My polish has a bald spot on her head from those instances and my silkies got bloodied before we got the second coop.
I strongly recommend against combining combative and non-combative breeds. Temperament has nothing to do with SOP, but so many stories of heartbreak have told me where which breeds come down. There are many extensive lists online or even just on BYC and there are too many commonalities between them to simply ignore.
