WCAG 2.0 is released should we follow only WCAG 2.0 guideline or we should combination of both no need to consider WCAG 1.0 now?
or if I'm considering WCAG guideline then do i need to consider any other guideline along with WCAG like RNIB, DDA Act, Section 508 etc. or if I'm already considering WCAG guideline then no need to look at other region specific guideline/act etc.
Is WCAG guideline alone enough and best to follow over any other guideline, for website from any region?
It depends on what you are trying to achieve.
BY satisfying yourself that you are compliant with the relevant WCAG 2 accessibility guidelines (A, AA, AAA), you can display the logo on your site, proclaiming your compliance to these standards.
However, if you are wanting to achieve a practical level of accessibility on your site, and are not worried about displaying a logo, then by all means pick and choose from the available guidelines to make the most accessible site you can.
WCAG 2 is the current edition of the W3C's guidelines on accessibility, and supersedes WCAG 1, however there is one particular WCAG 1 issue that I still feel is important: providing descriptive link text.
WCAG 1 states that links should make sense if read out of context. This means that you should not have links like "read more" or "buy now". The reason for this is that screenreader users will often browse a page using a links list, which just displays all of the links on a page. If there are multiple "read more" or similar links in a row, each linking to a different page, this will be problematic for someone who cannot place them in context easily.
WCAG 2 states that link text should make sense in the context of related text e.g. the previous paragraph or heading. This allows for links such as "read more".
While there may be situations where multiple links with the same text are unavoidable, such as on a shopping site (You may have a multiple "add to basket" links), I think it is wise to follow the WCAG 1 recommendation here first and foremost, and strive to have descriptive, standalone links on your site.
This is just one example, but does show that you can pick the most useful requirements from each set of guidelines. That said, I think WCAG 2 would provide the best base level of accessibility from the current available guidelines; you may wish to augment it with custom checkpoints of your own, but by just following WCAG 2 recommendations (and the user testing mentioned below), you should be achieving a good level of accessibility.
I think general consensus would be that, regardless of which guidelines you follow, to achieve optimum accessibility you should also be looking at undertaking user testing of your site, picking users with a range of disabilities to run through typical user journeys.
I've found that this nearly always turns up issues both of an accessibility and of a usability nature that just would not be covered by following the existing guidelines (WCAG1\2, DDA, 508 etc).
I have found this resource to be quite helpful when I made the transition from WCAG 1.0 to WCAG 2.0: http://wipa.org.au/papers/wcag-migration.htm
Whichever standard you decide to follow is irrelevant, as long as you understand that both will guide you towards more accessible content. Most of the times, the wording is different, but the intent remains pretty much the same. WCAG 2.0 does have the advantage of being an updated version of the standard that takes into account the evolution of web technologies so this could be considered a plus.
But if you decide to stick to WCAG 1.0, you can still achieve perfectly accessible results. Some people I know don't want anything to do with WCAG 2. Others have buried WCAG 1.0 the day WCAG 2.0 was adopted. Others like to play around with both. It's all good, as long as requirements are applied the appropriate way.
WCAG 1.0 and WCAG 2.0 are nothing but tools: it's what you do with them that counts.
Every little effort you put into your code, whether it comes from WCAG 1.0 or 2,0 will help users with disabilities.