what can i safly feed my amp?

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Hi everyone

Iv only just recently became intrested in making effects, and built a few projects from some net-schematics. Anyway, I want to start making my own sounds, but I need some help and would really appreciate any input you may have.

Basically I wanna use some SPICE application to lay out a simple distortion (or whatnot) circuit, tweak it with some simulations till I got the waveforms and frequency response to look "intresting", and then build it and see what it sounds like, maybe changing around components on both breadboard and SPICE to see and hear what I'm doing at the same time.
I have two problems - firstly connecting the output wires in my SPICE app. In real life these would go to the amp-in, but in the SPICE app I'm not sure how to accurately simulate a guitar amp's input to measure signal around.
I figured id hook in a resistor where the amp would go, and measure voltage on that, and it does seem to work (haven't actually built any of those schematics yet) but I'm not sure if that setup really gives me 'amp properties' to measure the output signal around and get a result consistent with voltage and waveform the real circuit would output. I'm also not sure what value to give the resistor, is it the amp's input impedance?

And a second question: when I design my circuit, what kind of output can I consider 'safe' to plug into my amp? or sound card for that matter? I know any DC bias is bad, but how much AC voltage can i safely feed my amp input? are there other issues that might damage equipment to look out for? wiring mistakes that could be fatal to the amp? and what about waveform, I  heard somewhere square waves, specifically in high frequency, can damage an amp (or the speaker itself?). Basically, once I'm reasonably sure my SPICE results are in tune with the real world, what do I look for in the measurements of the output to tell me a signal is safe to feed into other devices?
And what is the difference in that respect between 'line-in' and 'amp-in' inputs? I'm also thinking about doing all this with a basic pre-amp circuit, if that makes a difference.

anyway I see I asked quite a bit, so if your still with me - thank you for reading.