I have Canon 600D with Canon 50 f1.8 and Sigma 17-70mm f2.8-f4 OS Contemporary lenses. I am looking to upgrade to a Telephoto now, I am considering Canon 70-200mm f4 L USM (non IS), Tamron SP 70-300mm f4-f5.6 USM, Tamron 70-200 f2.8mm (It's bloody expensive though)
You've already got the portrait lens you need: The 50mm f/1.8. On your APS-C camera it provides an angle of view the same as if you were using an 80mm lens on a full frame body. That angle of view also means you would be shooting at the same distances and getting the same perspective to get the same framing as if you were using an 80mm lens on a full frame body.
The problem with describing a "good beginner's lens for portrait plus telephoto" is that it's hard to define how "good" is "good enough" when placed against "beginner" which usually means relatively inexpensive. With very few exceptions when it comes to lenses you get, more or less, what you pay for. Since every lens is a compromise between an ideal design and size, weight, complexity, and cost what makes a lens priced for a beginner is not the same as what makes a lens a good, or even great, portrait lens.
If your main concern in purchasing a telephoto lens is for portrait work I wouldn't worry about focal lengths longer than 200mm. That's about as long as you need for conventional portrait work.
Lower priced lenses that go past 200mm really start to compromise image quality throughout the entire zoom range in exchange for that extra reach at a budget price. The Tamron 70-300mm f/4-5.6 is such a lens. You sacrifice the wider aperture often desired for portrait work and you also wind up with a lens that isn't as sharp as the others you've suggested, while also demonstrating more aberrations such as CA, distortion, and light fall-off in the corners.
So get the best 70-200mm your budget will allow. The Canon EF 70-200mm f/4 L is a great lens for the price. The EF 70-200mm f/4 IS is a newer design and even better lens even apart from offering image stabilization. The EF 70-200mm f/2.8 L IS II is perhaps the best 70-200mm zoom ever made. If going the third party route, be sure you are getting the right version. The Tamron 70-200mm f/2.8 Di VC is a far better lens than the much older Tamron 70-200mm f/2.8 Di Macro, especially if AF speed is a factor for you. The newer Tamron is even marginally better than the older Canon EF 70-200mm f/2.8 L introduced in 1995 on the short end (about 70-100mm) while the Canon is slightly better from 135-200mm.