If you downgraded your game in order to run your mods, you may now have two executables in your Grand Theft Auto San Andreas folder, gta-sa.exe and gta_sa.exe.

the dash executable is the current Steam release of San Andreas, and the executable that Steam will always launch, the one with the dash in its name... While the underscore executable is 1.0.

All you have to do to make Steam launch the underscore executable instead, is to add this to your launch options.

"C:\Program Files (x86)\Steam\steamapps\common\Grand Theft Auto San Andreas\gta_sa.exe" %command%

of course, if your installation is somewhere else, you need to alter the path. Now, when launching from Steam, it launches your 1.0 executable and you can play with mods and have all the benefits of Steam!