When adding the rental income for Unit 2, it asked for % sq.ft. rented out.

A couple of things I would have done differently would have been to keep my W2 income as lonnnnnnnnnnng as possible, like longer than you think. 2nd would be do wholesale or sell more in the start rather than trying to keep everything possible as a rental. 

There are hiccups if the landlord hasn't completed their end of the paperwork or requirements through the online portal but nothing that was a crazy delay. 

So if you think you can net say $45k/yr in profit it will take you 3 years to break even if you ignore income taxes.

Prioritize constructing long-term wealth, striking a balance between immediate rewards and long-term objectives to generate passive income and lasting wealth.

I pay 6% interest only on free-and-clear houses, and have also taken over mortgages, subject to the existing financing.A rental ought to be on your tax returns for 2 years before lenders use the extra money as income.  

Note: Payment on any debt secured by virtual currency is an exception to the above policy and must be included when calculating the debt-to-income ratio.

How can I setup a Solo 401k if I have only rental income?

Fix-and-flips are active income.

Check out other online guides available here: https://www.biggerpockets.com/guides3.