If I get delta 1010Lt, or delta 44 or 66, do I need to buy a seperate software package? and will either of these three be compatible with a yamaha 16/4 mixer?
Any of those 3 will be compatible with the yamaha. They do all include a free recording application, but it's functionality will probably be very limited.
The stuff that comes with the Delta 44 isn't so great. (Reason adapted is pretty cool, but it's just a synth, not a multitracker.) Take a look at N-Track for a good cheap solution.