In some markets, supply remains very low, and prices continue to creep up with demand.

I am also curious to understand as how can one person can grow their RE investment if his/hers salary remains constant. i.e if someone is making 100k and has a mortgage and rentals, at some point in time the bank would not be able to finance the loan for a new rental.

If I am pre-approved for a 200k home but If decide to buy a duplex worth 150k would I be able to use the remaining 50k for some TLC.2.

try great western or what ever the name is of that outfit they will sell you over inflated props straight form the MLS

In a sandwich lease, the investor remains involved and on the hook as a principal lesee/optionee throughout the lease term.

Do I need to look at getting the seller to finance the down payment and then see if a bank will finance the remaining 80%? 

That means you need to have 25% remaining in equity. 

Let's say I want to purchase another single home for 500k which 174k remaining still in 25% sales price of the property.

Still, the question remains.  

The remaining 31 pads just have water and electric but need septic added.