i read that its 2.5 d , that all the objects is actually 2d sprites, is that true ?
You can use 2D spirtes, 3D models, or a combination of boths.
2.5D means different things depending on the context. For example, an otherwise 2D labyrithn with overhead passes or teleportation points is 2.5D. An scenario where the movement only happens on a plane but has 3D graphics is 2.5D. An of course, a fractal whose dimentionality is 2.5, is 2.5D.
Is any of that true for an isometric game? Perhaps. You can make a fully 2D isometric game, you can make it 3D with movement contrained to the horizontal plane and with a particular camara projection to get the isometric look, or you may not constraint the movement to the plane at all... at that point, is is not 3D?
Who cares, don't pay to much attention to "2.5D".
can we combine it (i use isometric camera, pan only, no rotation) ? ,i mean can we use 2d (for buildings) and 3d (characters) ?
Yes, you can do that. Both your buildings and characters can be either 3D models, or 2D sprites using billboarding planes. Use what you need. If you lock the angle of the camera and use orthogonal projection, it can work.
Using 3D would allow better effects, for instance, it can give you more realistic light effects. However, being the game isometric, you are already taking your artistic license on the presentation.
i recently play android games (Fun Hospital Tycoon), anyone know the game ? are they using full 2d or combined ?
How you can do the same effect? I would suggest to use full pixel art. Looking at screen shots of the game, I have no reason to believe that they need 3D models in runtime. In fact, doing it all 3D would probably have a performance hit on the average android device.