If I understand your question properly, you're asking if you can take a second vehicle as a FA in addition to the dedicated transport to transport a full squad of harlequins, right? If that's your question, I'm afraid the answer is no. Taking a transport means you're sacrificing bodies for mobility. You'll be pretty much guaranteed to be able to assault turn 2 if you so desire, but you'll be doing so with fewer bodies.
The dock strike means I haven't been able to obtain my own copy of the harlequin codex, but it sounds like the shadow seer powers are pretty fantastic for keeping squads alive as they cross the board. I expect to see a lot of armies doing a mix of transported and transportless clown squads.
Also, unless I'm missing something you should be able to take dark eldar as allies. That will unlock raiders for you (which can transport a full harlequin squad) and webway portals. personally, I love the idea of a big squad of harlies with shadowseers and jesters attached portaling in alongside their archon friend, putting up defensive powers, then rushing out to join the fray. You could even fit a ton of anti-vehicle weapons in that squad, go for rear armor on a tall vehicle, and then have the whole squad hang out behind the wreckage.
