There are plenty of youtube videos (and other sources) on painting portraits in acrylic.
You may just have to look at other body types while you are learning and once you have the skills to work on your own you can paint the subjects you are interested in.
The differences between the mediums is pretty profound. However theories of composition, the use of color, and the use of light and shadow to model form would apply to all mediums.
Yes - Acrylic and oil paints create different effects on the canvas, they are blended differently, and they dry at much different speeds.
Acrylic painting techniques are different styles of manipulating and working with polymer-based acrylic paints. These types of paint eliminate the need for turpentine and gestapo, and can be applied directly onto canvas.
Here are some tips for ** Oil Painting**.
They are different but you can still learn by watching video tutorials of oil painting.
Since I am oil painter (who has tried acrylics) I will share my experience and talk about practical part of painting with those. I am sure you know all about science behind it.
The biggest difference is the, so called, "flow" of the paint. It feels different. It absorbs differently on canvas. I would compare it like this: Oil colors stay on the canvas, while acrylics get soaked up by it. Which is to be expected, of course. That is the reason some people will prefer one over another. I prefer oils. Now, this, what I just said makes it all different.
If you are about to paint a portrait by using oils, you will have easier time with blending. Oil color dries slower. You can use this to your advantage to postpone blending of certain area for later and work on something else. It gives you more flexibility.
That flexibility comes into play when you are using, so called, glazing technique. Since it stays on top of your background color, you can manipulate with it, remove it and increase the vibrancy of your painting. Overall, in my opinion, glazing with oils is more effective and easier*.
*Why easier? Here is my story: You are not really limited by temperature. Once, during the art colony, I was given acrylics to paint with. We were outside in the hottest city in Europe - Mostar. My colors would dry in front of my eyes, so fast, that I could not blend them at all. Since I am using a lot of layering and glazing, this was a total nightmare for me. I pulled something off, but colors where dull and dead.
With oils, you can always come back, fix something, change shadows, lightning etc. Oil colors will cover it all.
Now, lets talk more about acrylics.
Since you are interested in portraits, your picture is pretty much set. For instance, if you were to paint something directly from your head, you would have to go back and forth a lot, fix something, change elements, add or remove something etc. So, you will not need your painting to be wet most of the time.
Faster drying means that blending must be done faster than with oil colors. So, there is the first difference. If a person is painting with oils, they can put darks first, then talk, then change something, then put undertone, then blend it. Why? Well, it is wet. :D They can talk for the next two hours and still come back and it will be wet. Some of them would place the basic color first (color the entire face with yellowish-red), then they would glaze, glaze and glaze to get the effect they want. For me, this just does not work with acrylics. It can be done, but it will not be the same. It is the best to glaze when under-color is semi-dry. Not too dry, not to wet. This is where I would say that you need to react faster than oil painter. But that is not a downside, really. You might actually like it.
For exact answer, I would have to watch every video and tell you what is easier to do with oils.
However, no matter what technique you are using, you can watch whatever you want and learn from it. You can even watch digital painting tutorials and still learn from it. The biggest difference is the approach and that is something that will come to you over time.
So, to sum it up, take into account: layering, glazing, blending and drying.
In short: Oil painter can manipulate with paint longer and thus change something that has been already dried on your paint.
One more thing is keeping your color pallet wet when working with acrylics. Also, you might need more color than the oil painter is using to get the same effect (because of how acrylics dry, get absorbed etc.).
Good luck and have fun.