Let's put it this way. The closer you get to PR, the more narrow the application will be. If you come into Singapore ready to invest 20Million in a business here, or you are the co-founder of Facebook, Obviously they are not going to cut their nose off to spite their face. So rather than say you MUST wait 2 years or 6 months, they leave it general so that THEY give out the PR if THEY want. Even if you wait two years, have two male offspring, a PhD and a desireable niche field of expertise, they can STILL reject you.
However, the realities of life are that while you CAN apply as soon as you get EP, the odds, if you are an employee are, for all intents and purposes, NIL. If you are an employee at the top of the food chain, you might get it in 6 months but again, a lot comes into play. If they think you are here to abuse PR, then no matter how well qualified you are, you still might not get it. The application is very easy to fill out. The requirement to apply for PR are simple. If you meet those requirements, it's just like the requirements for an EP. They are the requirements JUST TO APPLY. Doesn't mean it's an automatic given. Too many politics to go into, but suffice it to say, you don't need an agent or lawyer as there is nothing on the application that is subject to interpretation. Been there, done that.
But this is just my thoughts. You want to apply, please go ahead. You want to use a agent. Feel free to do so. The reason most agencies have good track records is that they will advise those who what to use them, that their odds suck or otherwise, and try to talk you out of applying as it will just bring down their success ratios. You want to use an agent? Tell the agent you will use him on Salvage Terms. No success, no payment.