Man... I really hate Canon's flash implementation. So I played around a bit with an optical slave unit the pop-up flash to trigger the remote flashes. In theory this works, but it's a bit hit and miss. Really not reliable enough to take to a shoot. (Even though I don't do any professional shoots I still want to be able to rely on my equipment)
So I went ahead and purchased two Yongnou 622-C and lo-and-behold once I attach them to my 7D I can actually select 2nd curtain! I thought I was finally set until I realized that this again only works if you use eTTL compatible flashes
My plan was to attach my Pixel Soldier on top of the YN to trigger all the flashes. But if I do that it's 1st curtain yet again. (Even though the camera let's me actually set it to 2nd curtain) Then I attach my Sigma 610DG to the remote unit of the YN and it does indeed go of on the 2nd. I try to connect my YN flash to the second unit and it goes off on the 1st curtain again.
Only upside so far I see is that with the YN I can actually remote control my Sigma eTTL flashes, which didn't work before. That means I can use my ring flash and 610DG in high-speed-synch and have the 610 remote triggered. That is pretty cool and the reason why I'll stick to the YN. In addition to that I can still use the optical slave and attach it to the PS and have it remotely trigger the two YN flashes I have. (Though then again it'll be a bit hit and miss, though better than the pop-up flash solution)
All in all really not nice and highly annoying
(Especially considering how much money I threw at this problem by now)