I have a 13 year old Percheron gelding. He's 18 hands and 2200 lbs. I'm 5'4" and use a 55 gallon drum as a mounting block. Not the most stable thing to use I might add.
Longing isn't necessarily that good for the big guys. I let my boy do a free longe in the arena. Not all horses need to be longed either.
You don't want to give the draft grain. The concern isn't so much making the draft hot (or sugar high from the grain) . Drafts should be on a high fat low soluble carbohydrate diet. The high fat diet can help prevent ESPM.
I wouldn't use alfalfa as the main part of the diet. Solon gets some for a treat but that's it. Good grass hay, a way to get the fat into their diet, water and a salt block are the only thing most horses need.
My draft gets the following:
A.M. 2-3 flakes hay
Noon 2 flakes hay
P.M. 2-3 flakes hay, 1/2 to 1 cup of Mazolla Corn Oil poured over packer pellets so he can get the fat in his diet and 2 scoops of MSM.
Two buckets of water, one for dunking, one for drinking
The packer pellets are a complete feed made by purina and the MSM is for his feet which were horrible until I got him on the supplement.
There are some horses out there that do need specific supplements. The best thing you can do is contact your vet and find out what would be good for your horse. You vet can also tell you if your area is deficient in certain things. Our area is selenium deficient.
We don't have round pens where I board, I just use the arena.
As far as fly control, I use bronco. Just your basic fly spray. You should rotate the types of fly spray. My vet recently told me that flies can become immune to the spray just like worms can become immune to pastes.
Worming - have your vet to a poop test to see what kind of worms your horse has or doesn't have. They can then tell you how often you need to worm your horse. I was following the Pfiezer every other month schedule and rotating the pastes until my vet said it was actually not good for the horse.
We did the poop sample and found out he only needs to be wormed twice a year, spring and fall, which is good.
You asked if someone is more or less likely to fall off a draft vs. a light horse and that is all dependent on the person. One horse is not better than the other when it comes to whether you're going to fall off. Two things come into play: how good is your balance and one breed has a higher drop than the other.
Pros and cons -
Everything is a pro, I'd never own a light horse again I don't think.
It sounds like you have a trainer lined up and that will be good. If your nervous, the horse is going to know that so you want to build of your confidence.
Since you have a cross, you probably won't have to much trouble finding tack. The bigger purebred breeds used to be a nightmare finding things but so many more people are riding drafts that more and more people are creating nice tack for the bigger boys.
You can find some nice stuff on eBay
Trailering can be a big problem. I bought a trailer while my boy was still 16 hands and he outgrew it in no time. I don't have a trailer now, but we have a nice trail near where I board and haven't had any need to go anywhere else. They are starting to make taller trailers now I heard so that may not be too much of a problem.
The best thing you can do is buy "Draft Horses, an Owner's Manual" it has everything you could want for feeding, illnesses, health care, everything. It's the bible of draft horses. Get the book and read it and you'll get a lot of fantastic information. Amazon
has the book here's the link: Amazon.com: Draft Horses: An Owner's Manual...