Difference between glasses
If there was no significant difference between “art” glass and “fusing” glass, there would be no point in spending all that money! Fusing compatible glass is that which has been tested by the manufacturer to be compatible when fired with any other of their glasses so labeled. Quite a bit of work goes in to determining the compatibility and keeping it compatible.
Unfortunately, texture, weight, and other factors cannot tell you which is which. You need to go by the labels on the glass. Spectrum 123 and Spectrum 123SF look the same, but they are not. They weigh the same, but one is fusing compatible and the other is not.
You will know that pieces from the same sheet are compatible with each other because the sheet existed as a whole previously. So you can put them together, but not very interesting. You can slump single pieces of glass, but you need to make sure the edges are the way you want them in the final piece, because the heat will not be enough to alter the texture of the edges.
So, in my view the extra expense buying and extra effort in keeping the glasses separate is worth it. It saves many breakages.
If you want to learn fusing with cheap glass, try float glass. You can buy coloured glass, powder, and confetti to match some float glasses. The coloured glasses are not cheap, but as the float is, the cost of learning is reduced.