There is a lot to know about inlay. Especially in different colored metals. Plain 24K gold is fairly simple, But from there on you should have some training from a master engraver as Brian said.
Advice from the web is often wrong and given by beginners who don't know either.
I think its this one entitled (multi colored and iron inlays} second page. Best wire is plain old iron wire from the hardware store. It can be bought in many sises. I have some down to .010.
http://fega.com/prodserv/DVD.asp?dvdparam=DVD&categoryparam=video&offset=25
I don't know any engraver better at this than Sam Welch. Years ago Sam did a seminar on this at the FEGA show. That was the best Seminar I have ever seen to date. I use to fill some small defects in castings with a tig welder but since then use iron inlays. All forms of bluing or browning...
I keep hearing some say the phrase "one slip and the engraving is ruined forever" or similar phrases. I have never seen a mistake that could not be corrected in a short time.
It is just a matter of learning how.
I think you will have trouble with the aluminum. The best thing to do is try it on some other piece of aluminum first. Same goes for the birchwood casey blue. Don't try it on the gun first. Acid is bad on aluminum but depends on the acid type. Hydroxide is even worse. Never heard of french...