Yes, you are absolutely correct that exhaust gasses in your car are a bad thing. That is WHY I'm saying you need to goop the plug bolts with high temp RTV. It will seal and crevices and solidify the nuts/bolts so that you don't get any gasses in the car or bolts loosening up over time.
That being said, I have worked in closed shops where some bonehead has fired up a car and not hooked up and exhaust hose to it. After politely punching him in the nose, I hooked up the hose. How did I know? I could smell it and it was a very clean running car. You SHOULD be able to smell a leak but with holes that small I highly doubt you'll have a problem that is beyond oh say, driving with your windows down or sitting in traffic (and this is if you do nothing to fix it). If you're moving chances are you'd never know even if you had a leak and did nothing just due to the cars aerodynamics.
It is OK to be concerned but if you do a good repair job, let the stuff set up, maybe pour a little water on it and see if it leaks first after its dried, then you should have no troubles whatsoever. If your cars in good shape and properly maintained you shouldn't have any leaks in the exhaust system itself either. Do the fix, water test it, no leaks you're good to go. Check it in a week or two after you do it just to verify your work to make triply sure you did it right.