This last one can be more complex, because certain types of income can be used and certain types can't. 

I would like to start with just one duplex, but I've been presented with this opportunity to buy a small apartment complex here in South Florida.

It's probably much more complex than you need and a Promissory Note and potentially a mortgage (if they are lending for a particular property rather than to your entity) would suffice.

That the rules are you had to live in it for a year and only 10% of the complex can be rented out.The problem is I have owned this condo for 7 years and been renting it just fine. 44/60 units in this complex are rentals.

However, some people make this work, it just adds a lot more complexity to the equation.

I’m originally from Plano and always looking to get back home with a 20+ units complex!

Storage is not rocket science by any stretch but if you are new to this, it might be advisable to master that before adding to the complexity.

I have an apartment complex in a college town that I picked up late last year.

Here's an idea @David Sweatman:If you have a 100k to invest then your personal residence is probably paid off.If you have good credit, use the good credit to get a HELOC at a great rate.Then use the money from your HELOC (and the 100k) as a down payment for a 20-40 unit apartment complex or commercial building.If you have an IRA with more 50k - you should consider using a self-directed IRA to invest that money in real estate too.

my answer is noEconomies of scale works better as you go bigger, this is why developers are building tons of large unit volume buildings in the area and very few duplex/triplex/quadplex or small complexes.