This forum thread should provider a little more information on the Customer type that can be used for loan accounts. Only identity Verified Customers can attach loan accounts.
The current Dwolla + Plaid integration only supports traditional checking or savings accounts (money market are supported as savings). Since ACH credits are only supported to loan accounts, I would recommend using the API or dwolla.js to submit the ACH bank details. Keep in mind that loan accounts require specific permission from Dwolla in order to be enabled. Typically we see loan accounts work in situations where you as the client have a direct relationship with the Financial Institution that owns the loan account for the user. However, as long as the financial institution which maintains the loan account enables ACH credit transfers to that account then the account can be attached to a Dwolla Customer/end-user.