Make your life easier.
Just use the DLs and keep the 580.
Use a hotshoe adapter like the link you posted.
Err yah, I would suggest don't anyhow try ... just to be safe.
Even a hotshoe flash has enough power in it's capacitor to possibly KILL an adult.
Studio flash has much more power.
Sometimes there are things that can be exchanged and try try, but for someone new to flash/studio flash, PLEASE DO NOT TRY ANYTHING other than the manufacturer recommendation or instruction.
Anything happen not my fault hor.
TS, i have the same lights... normally use these to trigger them... the pt-04's are relatively cheap, the reciever has a pc sync to connect to the lights and it lets you be a little more free with less wires connected straight to you...
btw, you do realise this whole time you could have just set your 580ex to pop upwards and use the photocell on the dlites to trigger them? not only do you get an extra flash to bounce from say the roof but you trigger the lights at the same time haha
set your 580 canon speedlight to Manual mode and adjust the power to minimum. bounce off the ceiling to trigger/fire your 2 Elinchrom units.
I reckon your 580 is in E-ttl auto mode and the preflash did trigger the elin chrom but did so before the shutter opens. so your picture did not register the lights thrown by the elin chrom.
all the best.