Hey are they any guides or rule of thumb for giving Pokémon extra moves?
For PvE or PvP? There are few 'mon that really benefit from dual moves in PvE but for PvP it’s usually essential.
GamePress did an article on this, though it hasn’t been updated to include Mons from Gen 5.
Keep in mind, this article was written with emphasis on PvP (where second moves are almost mandatory on a lot of Pokemon). If you’re looking to unlock second moves on Mons for PvE (gyms/raids) use, you’ll want to focus on Mons who can perform well in more than one role.
My personal recommendations in no particular order:
Chandelure - shadow ball/overheat
Roserade - grass knot/sludge bomb
Weavile - foul play/avalanche
Mamoswine - bulldoze/avalanche
Moltres - sky attack (legacy)/overheat
Blaziken - blast burn (legacy)/focus blast
Honchkrow - dark pulse/sky attack
Rhyperior - stone edge/earthquake
Swampert - hydro cannon (legacy)/earthquake
Mewtwo - shadow ball or psystrike (both legacy/ice beam or thunderbolt or psychic
Machamp - dynamic punch/rock slide (the extra move for Machamp is for attacking gyms, as rock slide hits quite a few Mons for SE damage that would otherwise wall Machamp).
I’ll put it this way, in PVP you’ll rarely find my teams not having extra moves. And there’s objectively good combinations of them but then there’s also your own strategy and things you have to experiment with.
As a note, some of these are a lot cheaper than others. For instance, there’s practically no reason not to give Blaziken and Swampert 2nd charge moves. They are really good in both their types and the unlock is very cheap. Others are a heavier investment (like Mewtwo and Chandelure). Same goes for Altaria and Whiscash in PvP.
Either PvP or PvE battle league is coming and rocket bosses are here now.
When you check a Mon on gamepress it never says about extra moves.
On the individual Gamepress guides for each Pokemon there will be commentary about their moves and how good second moves are for each, usually in reference to PvP. There are only a few raiding ones that are useful to have second moves on, usually if you want to power up one Mon to fill two roles (@BlameScott has a good list up above). Personally, I love dual move Weavile as they’re high tier ice and dark attackers, as well as Mamoswine (ground and ice) and Chandelure (fire and ghost). Most of the starters that have community day moves are no-brainers since they only cost 10k (Blaziken, Swampert, Torterra, Charizard, Typhlosion all have good uses).
Clefable - Meteor Mash/Psychic
Really, though, Clefable is about beating down stuff with Charm. The second move is cheap at 10K dust, so get it for coverage.
also for metagross, id do meteor mash and psychic (after the buff)