Welcome Trasd and glad to hear the interest in SFC!
I hold SFC as my favorite game because of the depth that you can have in a battle between two even players. There's nothing better than the multiplayer matches that last for an hour or more and are tense for every minute of it.
1. SFC1 is not as active as the others, unfortunately, but I do recall reading here about some players who did use it for multiplayer. The most-played version is SFC2:Orion Pirates, probably because it had the most potential to be modded. I don't have much Dynaverse experience, but I *think* there are at least a couple servers for campaing-based multiplayer games. SFC2:OP has the most players for standard multiplayer games on Gamespy Arcade and GameRanger (standard multiplayer can be as small as a duel or as large as a 3v3).
2. I still think SFC3 has some Dynaverse and Gamespy activity, but not as much as SFC2:OP, since it is a much different game from SFC1.
3. YEAH!
4. The tutorials are good, but there is no harm in messing around with simple skirmishes -- if you need to, you can give yourself a bigger ship and eventually give the enemy an equal ship as you get the hang of things. Do you know of a race that you have an inclination with? The Federation is the "standard" to start, but is in many ways more complicated than other races (the Gorn might be a good starter).
5. Marines can be tricky (the interface seems a little buggy sometimes), but it be sure to make sure that the facing enemy shield is down before sending marines, and that you are within range 5.
There has been some nice news about SFC2:Empires at War and SFC2:OP being sold here at Dynaverse, so there is no problem in getting a copy of OP if you are interested in playing OP online (OP is the stand-alone expansion and doesn't need EAW or SFC1 to run). SFC1 is very similar to OP, so I would recommend playing SFC1 for a while and see how it goes.
As a final thing, the SFC series isn't great with the newest versions of Windows. The online Dynaverse in SFC2 has trouble with Vista and Win7, but some experienced D2 players would know more about it. But since you're playing SFC1 and it's working, SFC2 should work (and standard multiplayer is more forgiving with Vista).
Good luck and let us know how it goes!