I would recommend using something from clerky and then getting your own lawyers involved to really nail this down further.
OP — for startups (as opposed to a regular new small business) in the US, what would be in a "founders agreement" is typically handled across stock purchase agreements, IP and confidentiality agreements (either CIIA or PIIA agreements), company bylaws, and Delaware law. It is pretty rare for US startups to have one single "founders agreement". I think the reason why you read about them is that they may be more common for regular small businesses, and some people consider any new small business to be a startup. For our purposes, to paraphrase pg, a startup is a company that is optimizing for growth (as opposed to distributions to owners).
And to answer your original question, yes, we (Clerky) are what most YC companies use to handle everything described above (the others typically have cross-border setups that require more tailored paperwork from a law firm).