Not an expert, but here are our experiences:
Not all bags are the same size. I like the medium sized bags. These hold +- 12 cups of rice and 4 or 5 bags take care of a 25# bag of rice. I'd rather have 4 bags to open than one big one, if one bag is bad I won't loose it all.
Not all bags are the same thickness. Seems like we have a few bags from almost every where we bought food from. We liked the bags the LDS sold us.
Our VAC-SEALER did not do a good job of sealing the mylar bags we tried it with. Many of the bags we have would not even fit in the sealer.
I use a clothes iron with a nonstick bottom and a round piece of wood, 1 1/2 to 2 inches in dia.
I fill the bag, drop in two O2 absorbers (and quickly seal the O2 absorber bag in the VAC-SEALER as quick as I can), and then curl the top edge of the bag around the wood and slowly lay the bag down. Might be able to do it with the bag up, but for me it works better to slowly lay it down and flatten it a bit. Then I run the hot iron the length of the open bag to seal. I roll the bag/wood slightly and run the iron again to double seal. There's usually plenty of trapped air in the bag so I can squeeze the bag and check the seal. By the next day the O2 absorber has "sucked" the air out of the bag and it looks and feels just like it was Vac-sealed. The sealed bags are labled and put into a plastic bucket for storage.
We've done several types of materials including rice and beans. Never had a poke through.