The two are not compatible.

You can use a FireStudio Project, however.

Not with an AI mixer.

Thanks for correcting me on that. I didn't realize that the AI could not daisy chain with the FSP as the non-AI does. I'll try to refrain from steering someone wrong on that in future.

All good. It's always good to have the questions other may answer so they can find their answers here...

Please help me. I feel I was indirectly mis-led by Presonus. When I was deciding to buy my 16.4.2AI, I was split between buying the older, less expensive 16.4.2 and the new AI. The older touted that it could clearly be expanded with the 1818VSL into 24 channels of recording. The AI literature never said yes or no. I opted for the AI.
I need to be able to record 24 channels. What should I do? Sell my AI and buy the older non-AI? Would I be able to use the Mac aggregate feature and use some other manufacturer's 8 channel interface? Is there a plan to make a Presonus interface that is compatible with the 16.4.2AI? I think it is ridiculous that I am begging for a product to be compatible with the same manufactureer's product.

The StudioLive mixers (AI and classic both) are FireWire interfaces. Neither one of them has ever been compatible with a USB interface such as the 1818. I'm not sure where you read this, but it is simply not true and never was.

The Classic StudioLive mixers could be cascaded with the FireStudio interfaces because they are both FireWire. They could also be cascaded with other StudioLive mixers.

The AI mixers can now be cascaded with other AI mixers, but cannot be cascaded with the classic StudioLive mixers or with the FireStudio interfaces. So your only (supported) option for more than 16 channels is a second AI mixer or a larger AI mixer.

As you mentioned, there is a possibility to MAYBE make something work with aggregate devices on a Mac, but it is not supported. As far as PreSonus's plans for future development, those are industry trade secrets and are rarely, if ever, released to the public.

I thought it was the 1818vsl but it was the 8 chan Firestudio. I don't care. Bottom line is I'm left out here with a supposedly better mixer that is not compatible with ANY presonus 8 chan. interface while the older mixer was fine.
Why can't you publish what your plan is for an 8 chan. companion to this mixer or for an update or fix to an existing interface? You early on published that you would have Dante and Thunderbolt interfaces which have not yet appeared. I think they are more interesting to your competition than you releasing, what is it, your 5th version of an 8 chan. interface. I'm not asking for anything new, I am asking for something old that is compatible with your own product.
I don't want to publish a picture of this mixer being tossed in a dumpster

You can't blame presonus for your bad research. I mean, you have access to all of the published info plus the forum to check stuff before buying into specialized configurations.

Also, you can take any ai-mixer and expand that up to 48(?) channels by cascading them.

Why did you not get the 24ai if you needed 24 channels? It should be approx the same money as an ai + 8-channel interface.

That aside you can probably get up to 23 channels on your 16ai.

16 normal inputs
4 aux inputs
2 two-track inputs
1 talkback input

If my memory serves me then all of these inputs are visible for recording in one way or another.