Shifting problems w/ Mavic Cosmos on 1971 Schwinn Super Sport
 

I just got this Mavic Cosmos wheelset for $120 off of craigslist, but am encountering a few small problems since getting them home. I guess I expected it, but I am hoping for help. Right now, I have the original "Schwinn approved" derailleur designed for a 5-speed freewheel *almost* working with the newer 7spd. I can't get the smallest 12t cog, not because of a lack of travel as I expected, but because (I think) this cog is too close to the chainstay. Changing limits and tension doesn't help the problem. Another potential cause for the problem is the poor chainline I have right now between the original Ashtabula crankset (which I plan to change out soon) and the 7speed cassette, which is outside of two small spacers that make it work with the 8/9spd Mavic hub.

Another thing I noticed about is that, when coasting, my cassette wiggles. It doesn't look like very much wiggle, but could this mean the freehub body is wearing out? At the previous owner's supposed 3-4 rides of up to 150 miles (which I mostly believe), this doesn't seem like something that should be happening on this wheelset, unless it came that way and is relatively normal
So what I'm wondering now, is should I just ride it and fix it when it's a problem or is there anything I can/should do now as prevention.

Finally, during minor truing before riding these wheels in earnest, there was a lot of 'pinging' going on. I don't really remember these noises when truing other wheels, but I assume they're not a problem if I can still adjust tension.