What capacitor values?

Mr H- No risk at all. It would bring the mid in at about 1kHz which is probably closer to the the neighborhood of the 12's natural roll-off. See if you can find the data on your 12". Often that will supply you with a freq response plot. Check it's 3db down point(upper) and design accordingly, if you don't want to use an inductor.

And a 3.5uF on top should bring the tweeter in about 7500Hz.
Is this OK do you think or should I be aiming for lower than this?

I'm not certain of the response of your midrange, never having seen the data. BUT- Most don't operate smoothly up to 7500Hz. If you look at this response: (http://www.madisound.com/catalog/PDF/scanspeak/12m_4631g00e.pdf) You'll note how erratic the response becomes after 5k. In the case of this mid: I would use it between about 700 and 5kHz where it's response is the smoothest. You may be able to find data for your particular driver, and choose roll-off points accordingly. On the other hand: Most tweeters are designed to function smoothly from somewhat below 5k to above(what's generally considered) audibilty.

The Mid starts to drop at 5K Hz as you say so I think a 5.6uF on the Tweeter would bring it in at 4740Hz.
Any risks in that?

Not at all. Just add the coil I mentioned in the previous post to roll the mid's top-end off, which will eliminate that peakiness from the reproduction and avoid a major overlap in reponses. Don't forget an inductor on the 12" to avoid the same in the woofer/mid region. It may take you a while to adjust to the new-found accuracy of the speakers and don't forget: The caps are going to take a while to sound right too(maybe 200+ hours of play-time).