And if we're okay with Western powers providing supplies, weapons, training, even actual manpower in shape of mercs and 'special operations' there -- how can we condemn Russia doing the same thing in Ukraine with a straight

There's quite a bit of a difference between the two. Russia more or less kickstarted the war in Ukraine, annexed part of the territory, deployed troops in the thousands, and are in direct and indirect control of swaths of the country's territory, while the government in Ukraine was democratically elected and is seen as legitimate. In Syria, there was an organic popular revolt against a dictatorship which replied with an incredible level of violence (especially compared to the Ukrainian government, for example). The level of involvement (and weapons shipped) also greatly differ; Russia has got tanks and BUK missile systems deployed in Ukraine, but the countries such as Saudi Arabia which proposed to send such level of equipment to Syrian rebels were blocked by the US and EU, at least as far as I remember.