[MrFlippant]

Since you mentioned digital, I'll make one thing clear...

You can run ANY car on ANY digital track... so long as the digital chip is of the correct kind. A car with a Scalextric Digital chip will not run on a Carrera Digital track, nor vice versa. But, you can install a Carrera Digital compatible chip into a Scalextric car, or any other brand of car. 

In some cases, it's plug and play, such as with the Carson DPR chip (which is Carrera Digital) into a Scalextric car that has the DPR hatch on the bottom of it.

In other cases, you will need to choose a chip that will fit in the available space, drill a hole in the chassis in the right location for the IR LED to poke through, and do some soldering of wires to connect the chip to the guide lead wires and to the motor, as well as secure the chip somewhere to keep it from bouncing around in the car as you race it.

The other thing I would suggest is to look into the options for making your digital track also an analog one. No, you cannot have a digital base and an analog base in the track at the same time and "only plug one in" as that may cause damage to things. What you want is a way to actively switch between the power that goes to the rails, from the digital base, or from an analog one. Another option is called "chipping the lane" which is not a complicated DIY procedure, and it allows you to use the same controllers for both digital and analog. With the ability to flip a switch and run one or two analog cars on your track, you can enjoy the wide variety of cars on the market without having to purchase and install a digital chip into them just to get some laps in.

As far as "will brand X car 'fit' on Carrera track" the answer is yes. Pretty much any slot car on the market will fit on a Carrera track without any need for modifications (other than the aforementioned chip, if the track is digital only).