True, though Slowbro and Jynx aren't really the limiting factors.
Think of it like this: the Psychic resist doubles their bulk, so fighting Slowbro, Jynx, and Rival's Alakazam is like fighting six Pokémon for the price of three. That sucks, but the Elite Four also has 23 other Pokémon. They're the ones who are really gonna drain your PP (though Rival's Alakazam can be a nuisance if it decides to Recover a lot).
Anyway, going back to the question of whether to catch something to help Alakazam, if you have enough Ethers/Elixers/PP Ups, go for it.
If not, catch Zapdos, Articuno, or some high-level Water-type (level 35/40 Tentacool is best IMO). With almost no training, they'll be able to significantly pick up some of the slack wrt PP. For instance, Tentacool should be able to beat Bruno, most of Lance, and a lot of Rival (Rhydon, Arcanine/Charizard, Exeggutor) by itself.
"I want to get off Mr. Giovanni's Wild Ride" - Zigman369