Grounding of ADC grounds
The answer depends on the resolution your are looking for in your ADC. For low resolution there is probably no need to isolate your digital circuitry from the analog ones(except the link itselft) and yes, connect all the grounds from the ADCs together and independent from digital logic. With more more resolution say 16 bit you must not connect the two ground planes together.
Quoting:

In higher resolution systems requiring more noise isolation, you might worry about stray digital currents flowing across the analog-ground region of your pc board. These currents can interfere with some extremely sensitive analog circuitry.
In general, when you encounter undesirable currents flowing in a particular pathway, you can address the problem through one of three approaches:
Reduce the level of the aggressive signal.
Disrupt the stray current by putting a high impedance in series with it.
Introduce a low-impedance element to shunt the stray current somewhere else.

There are problems which arise There is a website explaining the various approaches(google was your friend in this one)
Link to a set of problems and solutions regarding ADC bonding