Once I sell that I’m gonna convert to a fourplex and do the ADU.going through cash quickly and also using credit cards when possible like to purchase materials.

0% Business credit cards + hard money loan = 0 money out of pocket.

I have two banks that have done so as well as a credit card, and know of a couple of organizations that have as well.

It's likely a scam anytime they sound like AI or the grammar is weird or they use the word "kindly", if they ask for you to respond to them off-platform, if they say their employer will pay for it, if they ask if the dates they've specified are still open (when they can clearly see they are open on your calendar), if they ask for a video of your property (they probably want to set up a duplicate listing with your photos and video and scam others out of money), or if they want a long term booking and they have just joined the platform and have no reviews (could have been evicted from somewhere else, and hope a newbie will let them in with no background check, they'll end up not paying or credit card is bad).  

Since the funds are business credit, you wouldn’t have to worry about personal financial strain, and it allows you to keep your capital intact.I’ve seen investors use this method in various ways: some use business credit cards (with 0% APR for 12 months) to pay for renovations or bridge loans, others use business lines of credit to cover larger gaps.

Is there a way I can "co-sign" with one of my parent's credit cards and be on their credit history in order to boost my credit score? 

If you play your cards right, this could be a great way to build equity and add value to the property.

Risking your house on these kind of moves can become a literal house of cards as you scale.

Here's a good folder setup.Bank StatementsCredit Card StatementsProperty Management StatementsMortgage StatementsSTR Platform StatementsInvoicesReceiptsClosing Documents (Purchase/Refi/Sale)LeasesCheck & Deposit ImagesVendor ContractsW-9’s & 1099’s

Those old homes are such a wild card!