dude, remember from now on that there's a big difference between lowpoly game models and highpoly models:
Lowpoly models are like "what you see is what you get". Also any real time renderer that ever was, and probably will ever be, renders geometry divided into triangles. Quads and all other ngons are divided into triangles. So that's why in your low poly model is ok to have triangles alongside quads. nGons should be avoided though, in case your mesh will be smoothed (by this I don't mean meshSmooth, I mean that all polygons belong to the same smoothing group) because you won't be able to control its triangulation right, the software will do it for you and it will probably do it in a very nasty way. Ngons are ok on non-smooth surfaces though, you won't even know they're there.
On the other hand, when making high poly models, you always have to keep in mind that a mesh smooth modifier will be happening on top of that. So...what you see is not always what you get. And unlike game engines, mesh smooth algorithms hate triangles and with a vengeance. They're not that polite with ngons either. Mesh smooth simply LOVES quads, however. Feed it quads and it will obey. In other words, you must keep all your polygons 4-sided as much as possible. Triangles create shitty artifacts in the mesh, as well as ngons. Why? Because if you look at a mesh smoothed model's wireframe, you'll notice that all polygons are quads even if on the low poly mesh there were triangles or ngons. Mesh smooth will try to quadify the mesh by splitting every existing polygon into 4. When it tries to split a triangle into 4 quads, the results won't be very good. Same goes for ngons. You'll end up with pointy pimples and sharp edges where you don't want to on the mesh.
So yeah, be careful with that. But don't let all this theory constrain you. Always remember: the 3d program is your bitch. That pile of digital clay is your bitch. You control it, not the other way around. So when you want a certain shape, DO IT! Slice that bitch hard with edges in the way you want without worrying about topology and keeping all quads etc. Getting the shape exactly the way you want is the MAIN priority. After that you can worry about everything else, and almost all the times it will al solve by itself.
