Pokemon-go – Can you predict a Pokemon’s next evolutionary stage’s CP

Like Samthere said, you can delay indefinitely, however id recommend keeping one Pokemon high to take gyms with. Don't power it up more than you have to but keep it high enough to get at least a few gyms. The extra star dust and coins you get should be worth the candies and stardust you spend to keep just one Pokemon high. I usually get rid of anything with a CP meter under 90%. It's probably good to set a goal like level 25, otherwise you probably will delay it indefinitely. Lastly, go ahead and evolve any Ratatas and Spearows (or other really common ones in your area) you can for the experience. Their evolved forms max out pretty low so they aren't really viable later, otherwise leveling will get really slow unless you are hatching eggs left and right.

The quickest way I have experience to catch Magikarp is to locate a Pokestop next to a body of water. With the combination of a Luremodule and an Incense, I have witnessed people "fishing" for Magikarp. I sat there and a Magikarp was spawning roughly every 2 minutes