I think this comes down to opinion - and frankly, it can be easy (if you just trade over Pokemon or choose a few Pokemon which are well rounded and make use of TMs, etc) or hard (if you go for a full team within the game, or a monotype team, etc). Some gyms 1st time around have given people trouble (even if some have levels similar to GSC - new movesets and all = more challenge), and some of the Kanto Gyms are nothing to sneeze at either at times, especially if you don't have a type advantage or they use Double Team Hax... rematch teams can be harder although usually by then you'd have a team sorted, so if you match up well they shouldn't be that hard, for a good number of the gym leaders. And then there's Red who is now the trainer with the highest level Pokemon from all of the games, and if you go at him with a normal team of 6 and don't do any special training outside of beating everyone once beforehand, expect him to severly outlevel you by more than 20 levels.
As for the 2nd question - sounds like something that should be in the Help thread, but nontheless, you can rematch not only various trainers in Johto/Kanto who give you their phone number, but all of the Gym Leaders through that method, and the Elite Four who get new teams (which may include 3rd or 4th gen Pokemon) and boosted levels. Whether or not they are challenging is again dependant on your team, really - say, Lance will be easy if you have Ice Beam everywhere, but otherwise don't expect it to be that easy. For the re-match gym leader teams themselves, go here:
http://www.serebii.net/heartgoldsoulsilver/gym-rematch.shtml and the elite four here -
http://www.serebii.net/heartgoldsoulsilver/elitefour.shtml
For a rough idea, after beating Red my 6-Pokemon party (no legendaries) was about the mid 50's/a bit less, while the Elite Four rematch teams rarely has a Pokemon under level 60, highest being level 75, so I'll let you be the judge of that.
Don't see much further point with this thread then - it comes down to what you think, really, on whther they pose a threat or not, and people repeating the same stuff = pointless. Hence, closing.