Wow! If you're able to wear street clothes underneath a 1 pc, then your 1 pc is too big for you. They're designed to fit snug, yet comfortably over the body. If you go down, you can get asphalt/leather burn to your skin b/c the suit ends up moving around and causing friction if it's too big.
But, you should be able to wear something like UnderArmour pant leggings and a long sleeve spandex-like material that will wick sweat from your body but will also aid in removing your leathers even in hot weather. That's what I use and it works PERFECTLY.
+1,000,000
I only wear a light cotton T, short or long sleeve, underwear, socks and chest/back armor. Anything else and the leathers will twist around you limbs and body moving the padding and surfaces that are designed to take the punishment will not be where they need to be.
I also went with a 2-piece that zips together all the way around. That way I can take off my jacket when I stop for a break on a hot day. Yes it's rare for us to get a hot day in the PNW, but it does happen.