Well 1.2.3 has been out for 5 years now so there are less people playing, but there are a few servers that get quite a few people in during certain times of day. Mostly evening though.
As for the open beta. People just dont play it cause its to buggy. Heck it was never ment to be played like a full release. It was a bug test build. It was ment for people to play it and report bugs so the team could fix them. Didnt work out well so we stopped making open betas.
ECX comes with something called CCI (Core Compiler Interface) Basically its an easy way to code in new characters. Any noob can put in his own character, but if you want to give him transformation effects then youll need to know how to code in pawn.
In any case the big pack is made using the CCI. So its a character pack for ECX.
You can install multiple of those. JUst know that if at any time the amount of all combined models loaded is bigger than 512 the game will crash during the loading screen with an error.