What are bitcointalk.org current probation rules/period?
Each rank has certain restrictions that are removed as your rank increases. Newbies have the most restrictions. They can only post once every 360 seconds (6 minutes), can only have plain text in their signatures, PMs will have a warning saying that the person is a newbie and potentially a scammer, and a few other things. The restrictions for each rank are listed here: https://bitcointalk.org/index.php?topic=178608.0
Newbies can post in any part of the forum though. The newbie jail has been removed.
Is it true that you have to pay to participate on that forum?
For the majority of users, no, you do not have to pay to participate on Bitcointalk. The only people who have to pay are those who are registering from "evil" IP addresses. "Evil" IP addresses are IP addresses which many "evil" things have been done from. "Evil" things means that multiple accounts were registered from an IP address and were later banned from the forum. Accounts can be banned for many reasons, usually spamming. In order to register a new account from an IP address with evil points, you will need to pay a small fee. This is an anti-spam and anti-DoS mechanism.
Most evil points end up being assigned to proxies, VPNs, and TOR exit nodes as those are what spammers use to register their accounts to hide their identities. If you are not registering using one of those things, then you likely will not need to pay any fee to register.