What is the difference between 起码 and 至少？
Both 起碼 and 至少 mean "minimum" and "least". They both can be used either as adjectives or as adverbs.

最起碼 is valid, but because 至 means 最. 最至少 is invalid (just like "the least least" and  "the most least" are invalid).

至少 has a rarely-used meaning of "very less", which can not be replaced by 起碼.

至少 is a little more formal than 起碼. But in the legal documents, 以上 ("equals to or more than") is more widely used.