I believe it was WP doctrine to use WMDs from the beginning of an invasion of the West.
For most of the Cold War it was indeed. The WP figured NATO didn't have the conventional forces to hold Europe, therefore NATO would wind up using nukes, therefore it made sense to just go ahead and use them (edit: and chemical weapons) from the beginning for maximum effect.
That baseline began to shift late in the Cold War. Both NATO and the WP also had conventional-only plans since they knew crossing the nuclear threshold was a political decision.
Out of curiosity, how willing was the West to retaliate if the USSR/Warsaw Pact used nukes?
If the WP used nuke
S the west WOULD retaliate. Who would do it and how far they would go are psychological questions that can only be answered based on the NATO nuclear leaders on any given date and the exact situation. It has been suggested by some fairly reputable people that the use of a
single tactical nuke by either side MIGHT actually bring the war to a screeching halt as the politicians on both sides suddenly realize they're playing for keeps and need to talk to each other NOW before things get out of control. Neither side wanted global thermonuclear war (at least not when the other side had their fingers on the buttons too).
Please bear in mind that military staffs write up a massive variety of plans for every situation they can think of (well, and have time to study and write up). That doesn't mean any
given plan will be followed even if the exact situation it was prepared for comes up - history is full of examples. Politicians, especially early on in wars, have a tendency to say "no, do it THIS way." The existence of ONE plan also doesn't mean it was the ONLY plan. That the US had a plan to invade Canada doesn't prove they didn't have one to invade Mexico, for example.