Wire Size
 

I have a set of MTX components which are rated at 90 watts RMS. Will 18 gauge wire be enough?

More than likely enough. Most people use around 14 gauge, I have 12 on 160 watts a side.

14 gauge

IMO anywhere between 16-12 we will be fine 18 is a little thin.

when i got my amp installed he used the factory wire's. When i asked him why he told me that the factory wire is 10x better then any regular speaker wire and won't break in a few years.

^^ factory speaker wire is tough stuff. I used it in my install as it was much easier and much more convenient than running new wires. I did test it first to listen for an audible difference...none...at least at the power levels I'm using (110 watts to the mids, 30 watts to the tweets). Factory 18 awg is much stronger than typical store bought 18 awg. You'll have no problems with your set up.