Equinox is a analog summing amp using sub-d inputs. You will need a Sub-D/TRS snake, but even with that you will only get 8 in the equinox, it's a bit low knowing that the 1&2 will be use as DAW inputs with no summing. If you want to add more inputs you can do it by buying adats compatible converters and use the two adat optical outputs of the FF800. But this might cost you a lot of money. You will then get 24 in the equinox (with a M-16 DA for around 3000euros).
i just got my equinox and money wise i decided to sell my FF and get the SSL ALPHA LINK MADI AX bundle (with a pcie madi card) wich is the cheapest option, around 2000 euros. You get 24out/in analog, d-sub format (which means you can hook it up directly to the equinox with 3 sub-d/sub-d analog capable cables. You will need two other cable, one d-sub to 8 xlr male (for the monitoring part) and one d-sub to 8 xlr female (you actualy need only 4 xlr as equinox has 1 stereo out and two preamp outs, so you can make the 4 other inputs TRS or any kind you need).
If you want to stick to RME and get more channels this will cost you a lot more money as the option would be the M-32 DA (with 32 outputs!) but this is around 4000euros and you will still need to get a madi pcie card (rme HDSPe around 1350) and another converter for inputs (a M-16AD for around 3000euros), crazy money in the end...
i hope this will help you