Two things:
First, I would encourage you to not skip sizes. Since you are at 18g now, you should really go to 16g before going to 14g. This will benefit you and encourage healthy lobes.
Secondly, you're right, acrylic isn't meant for long-term wear. I would suggest you get something else in addition to the spirals to wear while a) stretching and b) everyday wear.
While you are stretching wear surgical steel, titanium, glass or quartz. I suggest quartz only because it is sturdier and, at smaller sizes, you can get plugs and other jewelry where I am unsure what the smallest size they make glass jewelry in is.
Once your stretch is healed you are going to want to wear some sort of inert material on the regular, I usually wear steel myself, as of late. When you want to wear something fun/flashy/what have you, that is the time to pull out your acrylic spirals. Generally people don't have reactions to acrylic in a single day of wear, but long-term your ears could react badly to acrylic. Also, spirals at such a small size could be quite fragile, so you definitely don't want to wear them while doing things that could damage them, including sleep.
I guess my point is, spirals are great during the day and acrylic is okay sometimes, but neither are good for sleeping in. ; ) In the meanwhile, you can probably get 16g steel spirals.
http://www.amazon.com/316L-Surgical-Ste ... B005SKP28M : D