LR atomizers are a way to simulate vaping at a higher voltage than your battery is capable of. There are 2 ways to make an atomizer/carto burn hotter and give a warmer vapor. One is to increase the voltage, and the other is to lower the resistance. If you have a battery that is only 3.7V, all you can do is lower the resistance.
The warmth of the vapor depends on the wattage consumed by the coil in the atomizer. Wattage is determined by this easy formula. Volts X Volts / resistance (ohms).
If you have a 3.7V battery with a LR atomizer (say 1.5ohms) the formula would be:
3.7 X 3.7 / 1.5 = 9.13 Watts. Most people like between about 7 and 10 Watts, so that's in the "sweet spot" for a lot of vapers.
If you only had a 3ohm atomizer for that 3.7 Volt battery, the formula would be:
3.7 X 3.7 / 3 = 4.56 Watts.
Twice as much resistance (3ohms vs 1.5ohms) with the same voltage (3.7) gives you 1/2 of the watts (heat).
A LR atty is hard on a battery though. They shouldn't be used on anything smaller than 450mah. It will kill a 220mah battery in pretty short order. So, you would need a larger battery or stick to a minimum of 2ohms and, preferably, 2.5ohms.
There is no difference in a LR and a SR in handling PG or VG. Since it gets hotter, a LR atty or carto won't last as long as a SR one. VG is a little harder on attys and gunks them up quicker. So, for the shortest atty life, you would use 100% VG on a LR atty. But it's not ridiculously short, so if you use 100%VG and your battery can handle it, go ahead.
I switched to vaping literally overnight, using a little 180mah 510 rig. I started with 100% VG. When I tried a 50/50 blend, I had some problems and found out that I couldn't tolerate more than about 30% PG very well. So, now I vape 70-80% VG.
Overall, I find VG easier on my body, but harder on my cartos and attys. The nicotine in vapor is mostly absorbed through the membranes in your mouth and sinuses. Strictly speaking, it's not even necessary to inhale into your lungs at all. I get occasional bouts of chest tightness when I chain-vape too much. When that happens, I just vape like I'd smoke a cigar. I inhale into my mouth, hold it a couple seconds and slowly exhale through my nose and mouth, involving my lungs very little in the process.
You need to make sure you drink plenty of water. PG and VG dry out your system. I'm not sure, but that may also contribute to that tight feeling in your chest. At any rate, it doesn't hurt and you need to keep yourself hydrated.