Invoices 8.1B is completely fine to install now. It will work with Receivables 8.1A or 9.0A. It is specifically the Receivables 9.0A upgrade that you have to wait for, until you have all the compatible releases.

In your case, BankRec is the only other program. Unless you choose to use the new 'accounting date' in AR90A to set a different GL fiscal period than the invoice/cash dates, in which case you'll need the upcoming Ledger 90B to use this feature.