Just a couple quick noobie questions about flashing roms

There has to be a way to tell how stable a Rom will be on your phone...

I had read about wanting the firmware versions to match? What about radios?
Also, how do I know if my phone is lollipop vs. kitkat? Isn't that important if you are S-On?
I'm asking because right now, I'm just booting Roms, hoping they'll work, just begging for a brick.

Sorry if this has been asked to death, maybe my search abilities are lacking. My reading comprehension is not, so links are always appreciated!
Thank you!

Couple of Issues

Hello, I have Samsung Galaxy S Glide AKA Samsung Captivate Glide, Rooted to 4.0.4 (Ice Cream Sandwich) with ATT Rogers Rom. Kernel Version is 3.1.10

Basically the problem that I have is when I am at school and have my WiFi on I lose connection. I have to keep turning on and off my WiFi because the orange down arrow stops as I only get upload then. I heard its because there are many routers at my school therefore it has problems.

Also when I log in to snapchat, snapchat minimizes it self and goes back to my app screen no error was giving it didn't even say snapchat has stop responding.

A couple of questions.

Hey guys, it's me again, this time with 2 very different questions! Hope someone knows this.

1) so, I had TWRP installed for a while on my device, and when I went to make a system backup, there was an option to "create system image" that I've never seen on any phone I've owned before. What is this system image and why is it such a large file? Is it a .img backup you can reflash with fastboot if something goes wrong?

2) this one is a simpler question. So I been reading around and I see that there are two fastboot commands to erase a partition. For instance "fastboot erase userdata" and "fastboot format userdata". Which is preferred and what's the real difference with these commands? Are they the same or is one preferred? When flashing a new stock image of course. Thanks in advance guys!