Conversions... what's the problem?

As Dan says, a major problem is that the bottom bracket is too low, add to this the short cranks and you have a recipe for serious ligament, muscle and knee joint problems as you inevitably spedn a lot time grinding - pedalling far too slowly. 90 -110 per minute and NO slow turning starts. I will be dead by the time all the fixed/singlespeed trendies are costing everyone a fortune in NHS payments but my kids will have to pay. So just don't do it.

I don't think that could be more wrong however hard you tried!

If you use shorter cranks you should use a smaller gear and spin at a higher rate (which is easier because of less leg movement) No damage to the body (in fact it could be less damage due to less movement required!)
And 90-110 rpm is not considered slow, in fact it is probably in the higher range for most people.
And don't worry about the NHS, those costs are will be taken up with the growing obesity problems.