[W126 Coupe] dumb electrical connections

[Shayegan, Richard]

Ok, I'm going to show how little I know about electronics here.

I found exactly what I need to do my ipod integration, no hacking away
at cigarette lighter plugs either. Blitzsafe UII. One end is an ipod
dock, the other is dual rca's and a constant power, switched power, and
ground cable that have no connector. So, here are my questions

I already have another ground spliced into the ground on my lighter. Can
I still splice into again, or can only so many wires go on one ground?

Do I still use rosin-core solder?

Same as the first question but with power wires instead of grounds. As
long as I'm not blowing fuses is there any issue with splicing power
wires?
Thanks,
Richard