From personal experience, I can tell you that Carrera slot cars are compatible with Scalextric systems. That’s if you are running them in analog mode. You don’t need any type of modifications to get them to work. I purchase a lot of non-digital cars and run them specifically on my Scalextric track in analog mode.
Digital Carrera cars won’t run on a Scalextric track unless you get a retro-fit chip. The issue is that you may have to do a little modification to get it to fit. The Carrera cars aren’t really setup to hold the chip so you may have to drill a hole in the body to fit the bulb. By default the bulb fits off to the side but to get it to work on the Scalextric it needs to be in the middle.
If you are going to try and make it digital I suggest trying it on Carrera slot cars that you don’t care if you mess up. I have a feeling it’s going to take some practice to get the bulb positioned correctly. I’ve done some digital modifications on a couple of FLY slot cars. Trust me, the first time you do it will make you nervous and you will make a mistake.