I'd think any attempt to put ematches in series would make them unreliable unless they're really consistently designed! Once one burns open, the circuit is broken and the rest get no current. If one is substantially faster than others the slow ones may fail to ignite. Homemade ematches are going to be less consistent and more likely to fail like this.
There is also a capacity issue. A photoflash produces high currents, but only quite briefly unless the capacitance is quite large (several in parallel will increase the capacitance). A pulse of tens of amps or more lasting only milliseconds may still be unable to heat the matches to the required ignition temp.
IMHO, it is also unsafe as well as unnecessary to use HV here. 250V is absurd. Ignition boards are often just an array of nails and a screwdriver with + voltage on it. Putting a photoflash cap there is just asking to get a major shock.
If they are all in parallel then one opening up will not affect the others.
A high series resistance from long, thin wire may necessitate a higher source voltage. 12V or 24V sounds plenty high though!
If your interconnecting wire is too thin for the thick nichrome you have, it may fail to work because the current might heat the interconnect wire up to a burning point before the nichrome will heat up. In this case voltage and current is irrelevant- any current high enough to power the nichrome will damage the wire first and higher voltages will only damage the wire even faster.