With MySipSwitch it is possible to: - register service with multiple SIP providers as client

This is easy once you understand the FreeSWITCH XML configuration.

- register your ATA with MySipSwitch

That's really a special case of the above.

- create smart dial plan (in Ruby) to route calls based on prefixes and/or call source and some other logic

FreeSWITCH can do better than that: there are lots of variables which can be matched by Perl-compatible regular expressions, including the destination number.

- accept and route calls from all registered providers

That's handled in the same way as the previous item.

MySipSwitch does not have any media capabilities - RTP is always established directly between call parties (no re-Invites either).

FreeSWITCH gives you a choice: you can use proxy media or bypass media modes if you don't want it to handle the media.

If this is possible - is there any beginner guide for this?

http://wiki.freeswitch.org/ is the best there is. It is being improved by the community over time. You can also take advantage of the IRC channel, the FreeSWITCH conference and of course the mailing list.