Nurses, please help this RT to better understand IV's and drawing blood!

Drawing blood sounds simple enough right? I'm sure it is and all...except that I'm an RT and not an RN.

Short backstory: RT has our own blood gas, chemistry, and co-oximetery machines in my hospital. As such RT recently took over processing all stat Chemistry and Lactate's plus VBG's as we have a 2-3min turnaround time vs. 1+ hour for the lab. This includes drawing them ourselves. Since we already insert/maintain A-lines and are allowed to draw off of PA and central/PICC lines we were also granted the ability to draw off PIV's if possible instead of performing a venipuncture.

Now the problems. My experience with venipuncture was "follow phlebotomy around and get 3 successful sticks and you're good to go". I also had no training in drawing off of PIV's - this includes in the policy the ability the pause/restart running infusions [Only fluids, only in certain circumstances, and only after conferring with the RN first] so we may draw from that site. Last time I learned about IV's and IV pumps was years ago in RT school. Fabulous start right?

I have probably a million questions but let me see if I can narrow it down to just a handful.

-When drawing off of an IV what should my thought process be as far as gauge size? Should I only attempt to draw off of 18G's? What about vein size? Also, should I flush the line with saline first to check the lines integrity before drawing?

-When it comes to IV's with running solutions I've been told I can pause the pump, wait a couple minutes, waste and draw my labs, and then restart the pump. I've also been told to never draw labs from any line which has been actively running electrolytes. Who's correct here?

-I'm straight garbage at successfully puncturing a vein. I can hit an artery anywhere anytime, but veins elude me always. Any tips?

-Any other advice you can think of or educational resources would be greatly appreciated.

Thanks!