1. Axon does not support the G.729 proprietary codec, or any other licensed based codecs. It does however support all the open codecs including GSM to my knowledge which provides the best compression rate.
2. As far as I know it is not limited, however I am still partial to the older versions of Axon which were free to use for an unlimited number of extensions so I don't know what has changed recently.
There may however be other factors that limit the number of extensions the system can handle, for example the processing power or amount of RAM available on the Axon server PC or the amount of bandwidth available to make simultaneous VoIP calls.
3. Generally if the soft phone is on the same network as Axon, there should be very little latency and the quality should be satisfactory. However, I have found that soft phones (in general) do not perform as well as ATAs or IP Phones for making VoIP calls.
4. Honestly, I can't say I would recommend Axon for a large corporation. I would recommend it for SOHO, Small and Medium Sized Businesses, but I wouldn't have a large corporation depending on its reliability, but that's just me.