If you're unsure of Basculin and Galvantula, the other options are Swanna and Zebstrika respectively.
Swanna can also learn Fly (if you don't want to use that on your Sigilyph), and is generally very good and useful and available right before the Ground type gym.
Blitzle is not only available super early in game, but also directly as Zebstrika right before the Flying type gym, so that's super useful.
As for your final team member, I'd Suggest Lilligant, because that would complete your Fire/Water/Grass core. Petilil can be caught fairly early, and with Quiver Dance/Petal Dance, you'll have a really strong Grass type Pokemon.