First of all, welcome to the United Kingdom. You will find that many things are the same as in India. But many things are different too and that is not just the currency that you earn in.
One of the things that are different is that most of the rules that you will face are not specified in detail. They just specify what a "reasonable" person would expect based on the evidence. That is the same basis for the jury system.
For a visit visa, what you need to provide is proof that your parents will have sufficient funds to stay in the UK and proof that they will return to India at the end of their visa period and not overstay. The latter is shown by strong tied to their home country.
So, let's look at your questions with this knowledge in mind;
RGA wrote:1. Do I need salary slips/bank statements for 6 months or are salary slips/bank statements for 2 months acceptable? Can I attach 4 salary slips from India and 2 from UK with corresponding bank account details (4 months India bank account and 2 months UK bank account)?
You need to demonstrate that your parents have sufficient funds to stay in the UK. If you have only been in the UK for two months, I would strongly recommend showing their own funds and additional funds, in addition to your salary.
RGA wrote:My parents have been married for 25+ years now and they don't have marriage certificate (as concept of marriage certificates was not present then). Their passports show spouse names properly. Is this sufficient or is marriage certificate mandatory?
India has had marriage certificates since the British Raj, but I digress. So long as the marriage is recognised by Indian law, the UK would recognise it. So, I presume that the last page of the passports may be sufficient. But remember that British passports do not list either parents or spouse or address. So, a British caseworker may not understand that to be sufficient proof of marriage. In any case, I do not think that marriage needs to be proven for a visit visa. I invited my mother and aunt to the UK and did not have to prove that they are sisters, only that I could support them sufficiently when they were in the UK.
RGA wrote:3. As a proof of them returning to India after 6 months, is there a recommended value of property (minimum value) which should be shown or just showing property ties sufficient?
This is likely to be your most difficult question. This is the most common cause for refusal of visit visa. You need to prove that they have such strong ties to their home country that they will return back to India at the end of their six months visa and not overstay. I do not think property by itself is strong enough proof. If you think of it, they could rent property out and live on the rent in the UK. Do they have other children in India that they will return to? Any other family? You will need much stronger proof in this area.
Hope that this helps start you off.