Two different things are getting mixed together here, so let me split them.
Class tracking is a software feature. You tag every transaction with a label, in this case a property, and the software can then produce a profit and loss statement per property out of a single bank account. "Classes" is the QuickBooks word for it; other programs call them properties, tags, or locations. It does genuinely give you property level reporting without separate accounts, so the first person you read wasn't wrong about that.
Commingling means mixing money that belongs to different legal owners. Your personal money and your LLC's money in one account is commingling. Two properties owned by the same LLC sharing one account is not, because it's all the same legal owner. So the usual line people draw is one operating account per legal entity, with class tracking inside it to split by property. If you later put each house in its own LLC, each of those needs its own account, and whether that separation actually protects you is a question for an attorney in your state.
The card in your wallet is the part that will hurt. Paying the plumber from a personal card and reimbursing yourself later is fixable but doubles the work, and if it's a mixed personal and business card you're now reading line by line at year end. One dedicated card for the LLC costs nothing and removes that entirely.
Security deposits are worth asking about separately. Some states require deposits to be held in a separate account, sometimes an interest bearing one, and the rule varies state to state.