Hi there,
I am not sure if you realize this or not, so i'll mention this...
A triac or scr and many ssr's do not turn 'OFF' unless the current
going through them falls to zero. In other words, if you use them
to switch DC current you wont be able to turn them off unless
you force the current to go to zero one way or another. Thus,
you can not simply send a signal to turn 'on' and turn 'off' and
expect the load to follow that and turn on and off as you planned.
Yes, it will turn 'on', but it will not turn 'off'.
This of course would be acceptable if you needed to turn a load
on and never turn it off until it's main power was turned off, but
otherwise it would not be acceptable.
Using a transistor, you can turn it on AND off, so you wont have
this problem. Using an opto isolator to drive the transistor you
can achieve total isolation for each circuit too. Choose an
opto with a high CTR (Current Transfer Ratio) so you dont have
to drive it too hard.
If you need details im sure someone could post a circuit.