1- Which OS I should use.
2- Which gcc& g++ I have to install.
3- In case that I already installed new versions of gcc & g++,
how I can un-install them.
Add 1) Any 32bits OS will do for ns-allinone-2.26 + ns-2.26-gcc410.patch.
The 32bits Ubuntu 12.04 that you have now will do fine.
Add 2) gcc / g++ version 4.1.2 .
You already have those packages installed. (Ref. your previous ns2 thread.)
You can have as many versions of gcc as you want, installed at the same time.
The available compilers all use unique file names : No conflicts.
( gcc g++ gcc-4.4 g++-4.4 gcc41 g++41 etc. etc.)