flex track is reagular track, with the only difference being that it is "flexible". This gives you the ability to make custom curves (since railroad tracks are rarly perfectly straight, or uniform in curves).
The only problem with flex track is that it will require some cutting. You can use a hack saw or specail "rail nippers" available at most hobby shops.
Specifically, you'll need flex track to make the 20" curves, since they are not available as sectional track. If you have the space, you can upgrade the one curve to 24" radius track, and then just keep the other at 22" instead of 20".
18" is to small to run the passenger trains you desire. you cars WILL derail. even on 22", my passenger cars have a tough time staying on the rail. The larger radius you can fit in, the better. I don't even think 20" radius curves will work to be honest.
your 18" curves would be great for a freight siding, or anything that handles locomotives equal or smaller than a SD40-2. Passenger cars are usually longer, and will not work on these, so plan accordingly.
I can't repeat enough, broad curves are key to passenger operations. I know, since half my railroad is based on commuter operations.