Yes, that's correct - or at least I can't think of a way it's not. The "some" you are referring to is the some that is used to define some unspecified or unknown thing. It is not the quantitative use of the word. It sounds a little dorky sometimes, but it definitely works.
It even works in this wack sentence because of how lot can be defined: I don't like a lot of people - I don't like some lot of people
Your second example is a bit odd. It should read more like: "I walked in the forest and met a/some squirrel." This is not dependent on changing "a" to "some", it just doesn't make sense grammatically to use it that way because the first action of walking is in the past tense whereas the second action of meeting the squirrel is in present tense. It could also be "I met a squirrel when I walked in the forest", or "While walking in the forest I met a squirrel".