FF classic chicken is a good choice for all your kitties, including Garfield. When Hannah was first diagnosed, we started her out on the Purina DM, as well. But we didn't have much luck with it. First of all, it's way too expensive... and second, she didn't even like the stuff. She's anorexic as well as diabetic, so feeding her has been a real challenge... but that's another entire story that I won't go into, here. Anyway, we've had fairly good luck feeding the FF classics. If you read the labels on the different flavors (LOL, hope your eyes are good or you have a magnifying glass), it will give the recommended number of cans to feed per pound of kitty weight. The can of FF classic chicken states "feed an average size adult cat one can per 3 1/2 pounds of body weight daily." But I'm here to tell you, from my experience, most cats won't eat that much and many don't actually need that much, either. I guess it depends on the kitty. Heck, it's a good day around here if I can convince Hannie to eat a can-and-a-half, and she weighs a little shy of 9 pounds and seems to do okay on that amount.
Now for your meter question. I use the ReliOn Micro from Walmart. It's a reasonably priced meter and does a fine job for us. It's a human meter, but it will do just fine for your fur baby. Lots of folks here use this meter. It uses a very small drop of blood, so that's something to consider when purchasing a meter for Garfield. It sells for around $15 in our neck of the woods. The test strips that sell separately go for around $36 for 100 strips. You can also purchase a smaller package of 50 for about $20, so the larger package is a better deal. You will also need a lancing device for about $5, plus lancets that go for about $4 for a box of 200. I have good luck with the ReliOn ultra-thin lancets, 30 gauge. As previously mentioned, prices may vary from one Walmart to the next. When I purchased all of these items for Hannah (almost 2 years ago), I was able to buy them as a kit. It came with a nice carrying case, the meter, the lancing device, a few test strips and some sample lancets. I don't know if the kit option is still available or not. One more thing... when we first started out, our vet's office manager talked me into a very expensive pet-specific meter. It was a great meter, but I couldn't afford to test Hannah as often as I felt necessary because the test strips were way too pricey for my budget... around $80 for a package of 50. Yikes! Since we were testing her several times a day, it didn't take long to use up a package of 50.
Sorry to be so long-winded, but I hope this helps... and best wishes.