My suggested order would be to start with just FS2 and the MediaVPs. Yes, you're playing the games out of order, but FS2 recaps the FS1 story well enough that you'll know what's going on, and it's a better game to decide whether you like this series and this universe.
If you enjoy it, try FS1 and Silent Threat Reborn. FS1 has some old school mission design, and feels a lot more dated than FS2, despite the fact that they're only a year apart, but it's still a lot of fun. You can skip the original Silent Threat entirely - it's buggy and poorly built - it seems to have been made on zero budget and inside of a few weeks. Some of the missions don't even have any events - no dialogue, no changes in AI orders, nothing. It's probably Volition's weakest release in the company's history. ST:Reborn is infinitely better, and brings much more modern mission design ideas to the FS1 era. It's a lot of fun.
From there, you have options out the wazoo. It really depends on what you enjoyed. If FS2 was really your jam, consider Derelict - it's one if the oldest user made campaigns, but it's fully voiced and holds up fairly well. Alternately, Vassago's Dirge is set during FS2 and is one of my personal favourite campaigns.
If you preferred FS1, consider Between the Ashes, which is also fully voiced and very well made, Bem Cavalgar, which is hard but very unique and very well FREDded - a true underappreciated gem, or (and full disclosure, this one is mine so I'm more than a little biased) Frontlines 2334, which is set before FS1, so no shields, and is quite a challenge.
If you want something more modern and dynamic in terms of gameplay, try Blue Planet, probably the most polished user made campaign of the bunch.
Beyond that you can go into some of the more unusual releases, like Transcend, which has a great plot but suffers a little from slightly limited mission design, or Procyon Insurgency, which can be frustratingly hard but is definitely worth the time to play through.