Authority is more important than Force
Individual police offers already face many of the same concerns you are bringing up here. For example, it is not uncommon for a female officer of average stature to have to face a male suspect who is twice her body weight and 3 times her strength. While you would think this means that she would have a harder time arresting a suspect, or put her in greater danger, you would be surprised. Most people simply will not resist arrest no matter how big or small the officer is simply because there is so much authority in being a police officer unto itself. The crimes associated with harming, resisting, or fleeing an officer are often much more severe than the crimes you are being arrested for, and in most cases are themselves enough deterrent to prevent a suspect from fighting back. In the vast majority of encounters that would be classified as dangerous, the police know going into it that it is a dangerous encounter and bring appropriate measures which would include superior numbers and if likely needed, weapons of lethal force. If the situation is dangerous enough, a cop will approach a situation with gun drawn, but the size of the cop /suspect are not supposed to be taken into account according to most police procedures when it comes to threatening or using lethal force.
Culture is also more important than Force
Gnolls are one of the respected founding races of aforementioned faction, not an oppressed minority, and therefore hold a good deal of sway
Police discrimination is real, but it's not arbitrary. We see this in the way that a cop's race does not have a big impact on how likely he is to discriminate against a certain group. In a given town, the police learn to fear/hate the same groups through past experiences and negative feedback loops of violence regardless of the cops race. When a police officer enters a "good" part of town to deal with a crime where he's never had to use force to get the job done, he will tend to be less on guard, and more prepared to solve the problem with words. Likewise, the people who live in that area tend to become more trusting of cops, and therefore more compliant. In contrast, if a neighborhood is of a higher threat, the opposite happens. When police profiling turns into police racism is where one race dominates one of these two types of areas; so, if your Gnolls are normally found in the more law abiding parts of town, then they will be more inclined to talk to and cooperate with law enforcement, and vise versa. (even those minority gnolls in the bad parts of town due to positive racism).
Another curious cultural factor is modern chivalry. Despite being smaller and weaker, female cops are less likely to be physically attacked than male cops. The use of lethal force against a female cop is exceptionally rare. This comes from having different cultural norms about using violence against women vs men. The majority of men in Western Civilization share the belief that it is much more wrong to hurt a woman than a man. This comes from the ideology that if you are strong it is your duty to protect those who are weak; so, attacking someone much weaker than you (or culturally identified as weaker) goes against most people's core ethics. Your Gnolls may share a similar ideology where humans are concerned. Thier history and folklore may be full of stories of valiant Gnolls use their superior strength to save Humans in distress, and that message becomes so engrained in them that when the time comes to talk or fight with a human officer, that they will be more inclined to talk things out.
Training is more important than Force
Up to this point, I think I've adequately answered the general question of how humans could still be an important and effective part of the police force without giving over an monopoly to the Gnolls, but there is also the implied question of what to do when things do get violent, and the reality is that police training already takes this kind of threat into account.
The first thing to consider is that a person trained in grappling is vastly more effective than a person who is not. When I was a in high-school, our wrestling coach decided one day to make a point about how important skill and training are so he made a bunch of the Freshmen face off against the Varsity team. After all us noobs got our butts handed too us in "fair" matchups, he went on to pair off 55kg guys with 80kg guys and 70kg guys with 105kg guys, and it did not make a huge difference. The experienced wrestlers won every single match. So, even when fights do turn violent, most of the Gnolls will not be trained fighters, just drunken idiots or what not, and the training of the human cops will still be adequate to overcome many significant physical disadvantages. Also, while that bite force the gnolls have is a scary thought, there are plenty of humans who work in animal control who "arrest" everything from dangerous dog breeds, to bears, to alligators. There are techniques to dealing with dangerous biters, and with proper training you can reliably avoid getting bitten.
Now the greatest threat where non-lethal force is called for is of course the trained, physically superior, and mentally ill offender. I used to know an Army veteran with PTSD who was on the upper limits of human size, strength, reflexes, pain endurance, and unarmed combat skill. So, I can confidently say he's about as dangerous as one of your more dangerous gnolls. One day he had an episode were he wound up outside during an episode of paranoid delusions. He was unresponsive to diplomacy, but his mental illness was obvious enough that the cops knew not to use lethal force, even though he was being clearly threatening. The 2 officers who first arrived on scene were warned that he was ex-military so did not even try to arrest him when they realized the situation was escalating, they backed off and tailed him from a distance but did not pursue closely enough to get caught up in a fight. Only after they had 10 officers in position did they attempt a take down. He was unresponsive to pepper spray or multiple tasers so in the end, all 10 cops simply had to pile on him at once with multiple offers immobilizing each limb. This whole story is to say that police already recognize when a perpetrator is too dangerous to go at alone, so when a physical threat is perceived as high, they use tactics in addition to their training to reliably win.
A final thought
Cops often have partners. Smart heads of police partner up cops who have complementary skills; so, it may be a very common pairing in your setting to see human and gnoll officers traveling in pairs. Let the smaller and less intimidating human handle the initial engagement to try to manage/de-escalate the situation, and have the gnoll ready to step in if thing start to get out of hand. Since your biggest concern seems to be not letting the police be dominated by gnolls, this sort of forced integration would do a lot to encourage mutual respect between the races in law enforcement while also minimizing how much racist cops could rally for exclusivity forming all gnoll clicks within police departments.