The lease is the instrument, and reading it is where the money is. The amount is only one of its terms. It also fixes the length of the commitment, what happens when the term ends, whether the amount can change during it, what it includes and excludes, the grace period before a payment is late and the late fee, what happens if you need to leave early, and how much notice either side must give. Late fees, grace periods and early-termination charges are contract terms rather than federal entitlements, so what applies to you is whatever the lease says, subject to state law.
Joint and several liability is the clause a shared lease turns on. Where several tenants sign one lease, each is ordinarily liable for the entire rent rather than for their share of it, so a landlord who is short by one person's contribution can pursue any of the signers for the full amount, and sorting it out among themselves is the tenants' problem. Where each tenant instead holds a separate lease for a room, that exposure does not arise. Which of the two you have is a question about the document rather than about the living situation, and because this is a matter of contract and of state law rather than of any federal rule, the lease itself is where the answer sits. It is a term worth finding before signing rather than after a roommate leaves.
The 30 percent standard is a convention with a paper trail, and it is worth knowing where it came from. The Congressional Research Service traces it from a nineteenth-century observation about what households were spending, through the Brooke Amendment capping what a public housing tenant could be charged at 25 percent of income, to Acts of Congress in 1981 and 1983 that raised the tenant contribution to 30 percent. Nothing was discovered in 1981; a programmatic contribution rate was legislated and then borrowed as a general-purpose budgeting rule.
The known weakness of any fixed percentage is not the one usually cited. Under a fixed ratio a household earning twice as much has exactly twice the money left over, by construction. The real problem is that needs do not scale, so the same 30 percent leaves one household comfortable and another short, which is why the convention is a starting point rather than an answer.
Rent is a fixed expense with a step in it. Between renewals the amount does not move, which makes it the most predictable large line in most budgets. At renewal it can move a long way, and the increase compounds against a salary that may not have moved at all. Treating a renewal as a scheduled financial event, rather than as a letter that arrives, is the practical difference between the two.
Paying rent on time generally does not build credit by itself. Landlords are not typically furnishers to the credit bureaus, so a decade of on-time payments can leave no trace in a credit file, while a single referral to collections after a dispute can. Services exist that report rent payments, and they are opt-in arrangements rather than something that happens automatically.