#1 - Kelley's products are inter-compatible, so long as you stay away from the rubber products. But, if you aren't sure, ask.
#2 - The Gunzite product has significant filling capability, but you may want to experiment with a gallon on a smaller area first. Remember that you MUST sand or roughen any primer that sits for more more than a week without being finished. The finish will NOT adhere well to fully cured primer. Also, the primers other than Gunzite do NOT fill gaps or roughness well.
#3 - I've seen some bad results with other paints. Kelley has more complete instructions than other companies I've looked it. I don't know that ALL of the products out there are inferior, but I've not been willing to experiment, personally. Kelley's products have performed as advertised IF (and only if) you follow directions. I *have* seen some severe failures with Kelley products when contractors did NOT follow instructions. In general, epoxy is not very forgiving.

On the other hand, plaster recoat quality is VERY dependent on contractor skill, and many aren't very skilled.