So, my final setup, that is working flawlessly is:
. 3 x Chromecasts connected to 3 Samsung TVs
. Main router TP-Link WR1043ND (DD-Wrt firmware) Cable Modem + 1 CC + Ether notebook
. Repeater router using wireless TP-Link MR-3420 (DD-WRT firmware) 2 CCs connect to this
. Core i3 notebook running Plex Server connected to the ethernet port of the main router and an external USB 2 3TB disk.
. My Plex "clients" are Android phones and tablets, Windows 7 and Windows 8.1
This setup can only play videos that have the correct tracks for CC (H.264+AAC).
What I mean is that the Core i3 380m, with 8GB RAM from my notebook lacks the power to encode on the fly, even if is only the audio track being converted, and so I prefer to do the re-encode myself before watching.
Some important points that I had to fix before having it working:
The quality of the Wi-fi signal matters a lot. The bitrate of most HD movies varies between 2000 and 20000 Kbps. Most routers don't go much further than 25 Mbits (even most of the 802.11n) in real life scenarios.
The CC can play mkv and mp4 (not only) but you must have H.264 video track with less than 5 ref frames and AAC 2 ch 48K, 128b.
Other "native" apps like Netflix, Play, Youtube are very good, no stuttering presenting a clear image without any artifacts and very good and easy to use. Youtube depends on the source video.
The available third party apps that support CC have bugs or are in beta stage, AllCast, Avia, LocalCast are some of the ones I have installed.
The exception is Plex, it is a complete package but for now it has some costs.
Plex Client is running in at least 10 android devices, and 5 notebooks throughout the apartment. Not streaming at the same time, obviously but all of them are mostly connected for sure.
So starting with a normal MKV (H.264 + AC3, DTS, DD+) the fastest and secure way to play this movie in CC is to repackage the MKV to MP4 by converting the audio to AAC (ffmpeg) in the process. Using ffmpeg, It will take less than 30 mins. and your movie is ready.