Yes, absolutely they can use nukes. There's this thing called low yield nukes. warhead size hovers around 10kT-12kT, a little smaller than the one's detonated in Hiroshima.
Russia has 6000 of those...
People has only asked what is the implication on Russia only, without asking what will be the implication on its immidiate neighbor. IF...and this is a big IF Russia detonated one on the field, the questions would be...what would the West do next ? Low yield detonation isn't enough justification to retaliate Russia in return with the same kind.
Russia using some low yield nukes in Ukraine is NATO’s wet dream. Such a reckless action would give them cover to offer any kind of offensive weapons to Ukraine, except for nukes, as the Russians would have escalated to a level that would make sending F16s or ATACMS a joke.
Also, this would force countries like India, Turkey and China to stop trade with Russia, which means a quick collapse of the Russian economy and regime.
Pariah state? They're already a pariah state
But they still trade with Turkey, India and China.
- Sanctions ? they're brimming with it, and recently surpassed NorKo
The sanctions against them are limited in scope in order to allow them to still sell oil and gas at lower prices to India, Turkey and China. There’s still room for much tougher sanctions.
- Countries severing ties with Russia ? With the exception of Belarus and maybe China, not one is important enough for the country at the moment.
You think they would do well if Turkey and India would stop trading with them?
- Military actions ? Could be, but escalations by two nuclear armed forces is something to be thought deeply especially if you want your house in Kentucky turn to glass just because some defenders in Donbass got nuked.
No need for direct military action if you give Ukraine cruise missiles and fighter jets. The Ukrainians would be happy to strike deep into Russian territory on behalf of the West.
- Countries start proliferating nukes ? That's the Western led world orders problem not Russia's. Imagine another 10-15 country the size of Pakistan or Israel and nuclear armed...that would be much fun. South Korea has made it clear that they're going to follow that path sooner or later, Japan might be interested to acquire one. In the end its the Western order that will have the most headache because they're not only dealing with "rogue" regimes (as they like to call it) but possibly allies that is breaking the norm on nuclear proliferation.
Yes, proliferation would most likely happen, but more countries having nukes will reduce the threat of war.