Choosing Basics/Stage 1s

Having More HP (While Being Searchable)

If you play Buddy-Buddy Poffin, you'll want your evolving Basics to have 70 HP so that Buddy-Buddy Poffin can search for them. 70 HP also lets them survive a Dragapult ex Phantom Dive while they're sitting on the Bench, waiting a turn before they can be evolved. If there isn't a 70 HP Basic, but there is a 60 HP one, you'll want that — at least it survives Dusclops's Cursed Blast.

Another rarer example is Girafarig: there are three Girafarigs in the format, but only one is Colorless and has 100 HP, so that's the one you want if you play Fan Rotom.

Having Useful Abilities (Or Not)

Some lower-Stage Pokémon have useful Abilities on them. Take Charmeleon PAF for example, whose Ability gives it protection from effects (like Phantom Dive, once again).

On the other hand, sometimes having an Ability can be detrimental. Having an Ability means taking damage from Froslass's Freezing Shroud, and getting walled by Cornerstone Mask Ogerpon ex.

So, which Charmeleon should you run? Answer: split them 1-1! They are conditionally useful in separate circumstances, so why not have access to both?

Having Useful Attacks

Lower-Stage Pokémon usually have weak attacks, but sometimes they don't, and it's worth playing those that have good attacks. The aforementioned Charmeleon MEW is one of them: Combustion 1HKOs Budew for a low 1 Energy investment, and Fire Blast is one of the deck's few outs to walls like Crustle or Cornerstone Mask Ogerpon ex (paired with Dusknoir to set up the KO).

Another example: two Gimmighouls have good attacks. The one from SSP can search the deck for 2 Basic Energy in case your Energy Search Pro is Prized, and the PAR 87 one has Nest Ball on an attack (but also a low 50 HP). Gholdengo ex decks will often run a 4-0 or 3-1 split of these.

Some strategies rely on having the "right" attack on a lower-Stage Pokémon for Relicanth to unlock.