Oh, I just reread your post and saw the part about the mini. Check out the post "Mini with a full sized horse" on the mini section. Its sounds pretty reasonable as long as they get along and the full sized horse has a gentle disposition (I want a mini too, but can't talk DH into it.). The bigger issue would be feeding. From what I've read, the minis tend to get fat and need portion control so it could be tough to leave out a round bale for the full sized one and not have the mini get obese.
The other thing to keep in mind is how well you and your in-laws get along and how similar your attitudes about animal care are. We had our horses at the neighbor's for 6 months - we provided feed and hay for ours and theirs, they were supposed to do the feeding,etc. He's unemployed and should have had plenty of time. I can't tell you how many times I found the water completely empty and 4 horses desperate for water! It was fine for the first few months when school was out and his 7 year old could do the work for him, but once the kid got busy with school, they never got fed, watered, etc., except by me. There were other issues there I won't get into, but just be careful. Make everything very clear about whose responsibility everything is, since you'll both have a horse in there. Can he call the vet on your behalf? Who handles cleaning out manure? How much hay would you pay for vs. the inlaws? Try to think about every detail of care and who will do what.
I was normal...then I got a horse.