Why two ( or more) gases have proportional volumes at constant pressure and temp.? (Avogadro's Law) I am in real confusion over Avogadro's Law..

In some of the problems I did on gas laws, there are the number of moles of the gas, the temperature. and the pressure. And it expects me to find the volume of gas.

But then in other problems there everything else is given including the volume of the container ( instead of the gas itself) and then something else is expected to be found. 

The confusion starts here. If any gas occupy the volume of volumes, always, then why will volume change according the combined gas law?

In one of my previous post, I learned that volumes of different gases won't be proportional (meaning all of them will occupy equal space) unless the condition of constant tempt and pressure is reached. 

But what really happen when constant tempt and pressure are reached? Do individual gases just separate themselves from one another? Because in order for their volume ratio to be equal to anything other than 1, their volume must be different.

you only have to answer the question in the title, the others questions are intended to help you understand what I mean. Sorry for the possible confusion

a link to my previous post
So, there are many questions, and I'm not sure if I understand you 100 %:

We have Avogadros Law, which applies for an ideal gas at constant pressure and temperature:
$$\frac{V}{n} = k \quad\Rightarrow\quad \frac{V_1}{n_1}=\frac{V_2}{n_2}$$
With $k$ as a constant.

And we have the ideal gas law: 
$$pV = nRT \quad\Rightarrow\quad \frac{p_1V_1}{n_1T_1} = \frac{p_2V_2}{n_2T_2}$$

And this is probably what your first question is about. There are problems when you have a state $1$ and a state $2$, which differ. 

For example: You have a container where everything ($p_1,V_1,T_1,n_1$) is given. Now you double the temperature - and the task is to calculate the pressure. Since the volume and amount of substance is fixed ($V_1 = V_2$, $n_1 = n_2$), you can just rearrange everything and calculate the pressure $p_2$.

In the ideal gas-model, gases do not seperate. I may be misunderstanding you, but if you put 1 mol $\ce{N2}$ and 2 mol oxygen in a container, you will have different partial pressures, but no demixing.

A relevant question was posted on physics.SE, that may help you if you have to dive deeper into this topic.