Hi Scott - welcome to the forum.
ARC Pro does have an analogue mode (running all the ARC Air features) as well as full digital. My experience - and the experience of other ARC Pro users - is that chipped cars do not run well in analogue mode. So analogue cars can’t really be raced against digital cars - so it is not a 100% perfect solution to the analogue/digital split. However, it Is easy to work round that. My personal feeling is that the analogue mode is not quite as ‘good’ as a standard analogue powerbase or ARC Air. Other people don’t notice any difference (apart from an audible ‘whine’ with some cars), so maybe it is a case of try-before-you-buy. I actually find myself using analogue mode on ARC Pro more and more - I‘m obviously getting used to it.
Whether you should add ARC Pro to you digital set depends a little on how much you have already added to it. If you have the C7042 Advanced Powerbase, plus USB dongle and PC race management software - then you already have something similar and possibly better than what ARC Pro offers at the moment. You could also add pit lane sensors and wireless controllers. However, from scratch that set-up costs a lot more than ARC Pro. If you have the basic 4-car digital powerbase, ARC Pro will be an awesome upgrade!
If you don’t already have a digital pitlane, add that to your shopping list. If you have curved lane changers, these will need to be modified to use with ARC Pro. You will also need a compatible smart device to run the ARC app and ideally a tablet to run ARC Pro. There’s more info on this thread: https://www.scalextric.com/uk-en/forum/arc-app-device-compatibility/?p=1
If you want to add different makes of slot cars to your fleet, almost all 1/32 scale ‘hard body’ cars will work with few modifications, or none. You mention Slot.it cars - they will run unmodified in analogue mode and if they have a chip fitted will run perfectly in digital. Some models are sold with a Slot.it SP15B Scalextric Digital chip installed. That chip is designed to convert Slot.it cars to Scalextric digital and are also sold separately - installation requires two wires to be soldered to the motor. Other brands of car - for example NSR and Scaleauto - are fitted with coloured guide blades, which do not work with ARC systems as the lap sensors can only see black guides. The coloured guides can be blackened with a good-quality black permanent marker or, ideally, changed to black guide like the Slot.it CH10. Carrera cars are designed to run on bigger 1/24 scale tracks - with a change of guide, they run fine on Scalextric track, but the supplied alternative ‘shallow’ guide is red! Black marker pen can be used - or BRM sell their black S-126 guide to convert Carrera cars to 1/32 track systems like Scalextric.
I hope that helps.