Everyone can read power levels it came free with spiritual pressure sensing
I forgot to answer this yesterday.
I don't think all characters can read power levels. If everyone can read, what's left of
Pesquisa?
Yammy miscalculates despite having Pesquisa. He couldn't calculate the power of an ordinary person like Tatsuke and asked Uluqiorra "Is that girl strong?"
Grimmjow still believes that he can kill TS Ichigo despite seeing him. If he could read power levels, he would know that it's impossible.
And it's not just Grimmjow. Many characters in the series think that their opponents are weak. Think of the characters who tried to kill Yamamoto and died. If they knew how strong he was, they wouldn't attack out of fear. The same goes for Zaraki.
Just because a character can see spirits doesn't mean they can read reiatsu levels. Unless there is concrete evidence that a character can read power levels (like having Pesquisa, for example), we shouldn't accept it.
According to the wiki, in order to use Pesquisa, it is necessary to close your eyes and enter a meditative state. Since Grimmjow did not close his eyes and enter a meditative state against TS Ichigo, it is not considered that he used Pesquisa and indirectly read his power level. Another way to use it is to read his power by placing your finger on the ground like Nnoitra did. It has also been stated that Uluqiorra is the best Pesquisa user.
Ok Clyde... There are no statements which say Urahara can't get stronger. Urahara at the start of the series was gonna die to Ichigo's first getsuga.
I think this is expressed in English as "Not being all out". I don't know how to express it since it's not my native language.
Urahara, I'm sure Ichigo said that because of Ichigo's instant power increase. Why would he need to be Full Power against someone he's already trained with? He says his worst arm got torn off. Besides, if a character is not in combat, that is, if they are "off guard", their stats will drop anyway.
A few examples
1) Tosen gets hit unexpectedly by Hisagi and loses the fight. In that scene, Hisagi thought he was defeated and out of combat. Considering that Tosen has at least 100 times more power than Shikai Hisagi, it's very possible that his durability would drop when he's off guard.
2) There's not a huge power difference between Luppi and Grimmjow. Despite this, Grimmjow one-shotted Luppi. Because Luppi was out of combat and thought Aizen would protect him from Grimmjow.
Urahara also sees Ichigo's Getsuga Tensho for the first time. He didn't expect this level of power.
Unless Urahara says something like "I'm rusty, I'm not as strong as I used to be", we can't say he's weakened. Yoruichi lost her Zanpaktou and started bulking up after the battle with Yammy. But Urahara didn't say he's weakened, nor did he train until Fake Karakura Town.
Edit: And I don't know what "Clyde" means
For Gin, I would say Gin>Harribel. That's because Aizen said let's go to Gin and Tosen after killing Harribel.
Let me present an argument that I haven't presented before.
The 3 arrancar girls next to Harribel use Resurection in the arrancar saga and summon Ayon. In the fight against Quilge, they summon Ayon without Resurection. FKT Ayon>TYBW Ayon. Ayon was beating Quilge. Quilge was weaker than him unless he absorbed him.
Harribel>Resurection Ayon>Wihout Resurection Ayon>Volstandig Quilge~~Fullbringer Bankai Ichigo
I'm currently reading the CFYOW Novel. From what I've heard before, Liltotto is afraid of Harribel. If it's true, the scaling is as follows:
Harribel>Base Liltotto>Meninas>Robert
and Bakuya uses Bankai against Robert. He feels the need to back down against Candice. According to the profiles, Candice got the Ap feat over Byakuya. It was also stated that Liltotto and Meninas were stronger than Candice.
Liltotto is a very powerful character. One of the strongest non-elite quincy
Royd>Gremmy>Bambietta>Quilge>Liltotto
The statement he used for Yammy should not be interpreted as just power. Zaraki stands out with his brute strength, while Byakuya stands out with his nobility. In the CFYOW novel, when Hisagi was compared to other characters, it was said that he was "not as noble as Byakuya" and "does not have Zaraki's brute strength". I do not think it is right to interpret it as just power.
Also, can't such statements be an exaggeration? There is a statement for Grimmjow that he surpasses his own race.