First of all, we're looking for optimal itemization. There are no such thing as builds for lower leveled games unless referring to snowball items.

Second of all, I wasn't even referring to Renekton + Xin. Xin rarely uses the CC on Fromal even if this is in a lower level game.
Think of it. With health + CDR, Xin E's in, procs his slow, lowers the cooldown on his E with his Q and then is able to E again.
With Fromal, Xin E's in, procs his slow, but leaves the enemy prone to using any gapcloser to escape.

Health + CDR is more reliable as a slow AND gives more tankiness.

Renekton-Xin is the magical combo now because they are both very good in diving an adc or apc and they are tanky enough to get out afterwards. They synergise well because Xin can dash in and keep the target busy so Renekton can come in and stun him and that will result in a dead target. In this case you're right about Xin not using his fromal cc. If you go to a standard gold league game this doesn't work that way. There you shouldn't expect a Renekton-Xin combo or even a dive-in buddy to begin with. ( Teemo's everywhere :p) Here a Frozen Mallet would be really useful. Builds at MobaFire aren't aimed at the top-players (they know what to do :P) and you should expect other tactics. The only reason why AD-yi builds exist is because it works (if you go really low on the skill-ladder), not because it's viable.

Always R>E>Q>W

For sure

you should always prioritze E over Q and get as much cdr and health you can probably because someone already said his ultimate grants him huge armor+mr per enemies hit. Mallet is good but it's not core and it's not cost effective.

Something I have to say here
It's my way of playing:
R > Q > W > E
Of course to me at least and i dont really use fromal on xin zhao, theres no need for it.

Warmog's Armor may only have 300 more health than Frozen Mallet, but it's also quite a bit cheaper. In terms of defensive stats, it's far more cost-effective.

Also you might not buy it for the regen, but it's a rather useful stat. In general hp5 isn't a combat stat, but warmogs regen is fairly significant. A bit like chugging a pot during a fight. It can end up giving you some more effective health to work with unless you're bursted down in a couple seconds. It's also nice against prefight poke of course.

Xin Zhao actually does benefit from a lot of raw health due to Crescent Sweep. At max rank he gets 25 armor and magic resist per enemy hit.

And the thing is Fromal doesn't help in teamfights as previously said, and is not even bought early on so it won't help with ganks.

The regen isn't even why you buy Warmog's. Warmog's is bought to survive in teamfights.

Fromal is like an item in between utility and defense that doesn't give much usefulness to both, so there's no reason to buy it.

Didn't watch LCS because couldn't catch the matches.

And the health isn't all that much more. Regen is great for clearing, but is useless in ganks and team fights.

Yes mallet provides those. So what? Xin doesn't use the fromal cc, and the health is much less than Warmog's.

And are you serious o_o

Xin is one of the top jungle picks right now in competitive play.

Mallet provides flat health as well, and CC. Warmogs provides flat health only, which is not as useful.

I don't think I've seen a pro level Xin since forever. Can't remember what Diamondprox builds on him.

There is no bruiser who doesn't benefit from flat health.

Fromal as an item is just a waste of a slot.

There's almost no situation where you'll need to continuously slow an enemy aside from your E to let your teammates follow up.

Having a lower cooldown on your E is what Xin likes. It's a much more reliable slow, does damage, and gapcloses as well.