You don't explicitly say in what person you intend to narrate the scenes told from the detectives' view, but your comment that writing (the antagonists) in first person would be hard seems to imply that you would like to write the detectives in first person and switch to third person for the villains.
Of course you can write different characters all in third or all in first person, and of course you can use both first and third person in a novel. The less banal question then is, what effect will such a switch from third to first person have?
When you narrate all viewpoints from the same person, either third or first, the impression is that all those characters are given equal weight. The effect is that of an ensemble cast of multiple protagonists all of whom are portrayed with the same level of detail and intimacy. This effect is most pronounced in first person narration, hence, probably, your hunch to switch to third person for less central characters.
In every narrative viewpoint, all other characters that the viewpoint character encounters are told in third person. Here is an example told from Marcy's point of view: "As I came home, my neighbor, Mr. Williams, greeted me. He told me he had a parcel for me." Here, the neighbor, who is a side character for the viewpoint character, Marcy, is narrated in third person. If the next chapter is told from the viewpoint of the neighbor and it switches to first person, the neighbor becomes a protagonist of equal importance to the viewpoint character of the preceding chapter, Marcy: "I gave the parcel to Marcy. She looked excited and I wondered what she had ordered. Was it the new toaster that she and her husband had been fighting about?" If, on the other hand, Mr. Williams' chapter is narrated in third person, the effect is that Mr. Williams remains in third person and therefore remains more of a side character: "Mr. Williams gave the parcel to Marcy. He wondered what might be in it and decided to spy on her later that night, to find out." Such a switch in grammatical person also allows you to change how much you get into a character's head. In the brief examples I showed the perceptions and thoughts of Mr. Williams directly in the first person narrative but kept more of a distance and only summarized (i.e. told) his thoughts in the third persion variant.
From these reflections it seems to me that it would be quite fitting if you narrated the viewpoints of your protagonists, the detectives, in first person and switched to third person for less important side characters whose viewpoints are told from more distance and who get much less "screen time".
A switch to third persion would also allow you to keep their intentions secret quite naturally, because you wouldn't get into their heads as you would in first persion narration, where, consequently, you would have to disclose more about them than in third persion and where keeping a character's intention from the readers often feels artificial and like a cheap trick.
Commonly a switch to a different viewpoint character is done with a chapter break, if it is longer (a page or more). If you switch to a side character's viewpoint only briefly (less than half a page), you might do it as a scene set off from the main viewpoint by a blank line before and after it, but chapters can be as short as you want and there is nothing odd about a chapter of only one sentence, so even brief switches can be their own chapter.
