Yeah max CP depends on your level, my Squirtle is the max CP I can have right now so it is impossible for me to catch one with a higher CP, when I lvl up I definitely can but I will just power him up back to max CP
My strategy now is just catching and hatching eggs all day, I transfer over every pokemon I get and squirtle is the only one I keep, when I reach a relatively high trainer level I'll have tons of candy for each pokemon and a lot of stardust since I hatch 9 eggs at a time and get it in bulk, so I'll catch them at high CP and evolve them from there
I think with any of the starters your best bet is just to power it up to max CP, finding them in the wild is pretty uncommon I've heard and you often have to hatch them from eggs (2ks) so it's easier to manually get a high CP rather than just catch it like that
Height and weight also play a pretty important role it seems, my squirtle is XL for both so it makes sense to focus on it instead of one that has a smaller height/weight, should all take this into consideration when choosing which to pokemon to transfer or keep.