can I get another 8 channels of ad/da on imac

Here is my question if anyone has any thoughts. I have an MR816csx with an RME adi 8 running into its adat in/out.

Using a 2009 Imac with 8 gigs ram and Cubase 5. I am wanting to look into how I can get another 8 channels of straight ad/da converters with my set up.

My concern is this:
Would another purchase of a Mr816 and 8 channels of extra converters via adat lightpipe run on an imac?
The Mr816 would have to be daisy chained as the imac only has one firwire port (and I have to run on a 400-800 cable).
Do I have any other options?

One last concern is clients that have been asking me about protools. Now that this seems to be an option with the mr816 how far down this road can I go (2 mr816, with additional ad/da converters via adat lightpipe) before it’s a real headache if I want to be able to track using protools?

Thanks for any and all advice.