I haven't had any problems shooting .38 cases in the longer chamber so far. It should not be a problem if you keep an eye on your chamber and clean thoroughly before shooting with .357 cases. Even if you do, as long as you haven't shot a great quantity of .38 it should not pose any problem. Just keep your chamber clean. As far as shooting heavy bullets in .38 cases you run into problems with case capacity versus pressure. DO NOT assume a load is safe unless you take it from a manual and even then, start at least 10% below max. and work up carefully, looking for signs of pressure. Ask yourself what you want a particular load to do. .38 loads are good for low recoil, mild report, can be very accurate for small game hunting or teaching someone to shoot, very economical to reload, etc.. The .38 works best with fast to medium pistol powders, and you will run into dangerous pressures real quick with any bullet and especially heavier bullets. My personal recommendation would be 90-160 grain bullet weight.
Don't stop asking questions! It's how we all learn. When I first started reloading I became a pest at the local gun shop and read everything I could get my hands on. If you don't subscribe to Handloader and Rifle magazine you are missing a great resource. Enjoy your journey.