This isnt to be rude, but I dont think you should worry about them having two boys, OR how often they eat out. Im my area, you would be paying about $100+ PER mini to do selfcare board, where you supply all of your own hay grain and shavings, clean your own stall, and feed your own horses. At the 5 horses you have, that would be a minimum $500 a month, plus the expenses of feed, hay, and shavings. Oh, and boarding barns around me dont allow you to board stallions, the ones who do are extremely expencive. They let you keep your horses there for free when they could be renting them out as self care stalls and making a $500 income a month. Do you use their water to water your horses? Electric for the lights, or even fencing? Utilities to run a barn and upkeep are expences too, its not that your horses are just there and cost them nothing because their home is paid for. Maybe the horse they gave away they thought would eat more than $100 worth of hay and grain before it sold, and it was better to cut the loses. To be honest, if that was their thinking, they were probably right. I used to self care board my horses, and I did clean stalls to cut costs down. I cleaned 10 big horse stalls a day, plus my horses stalls. I paid $70 a month for my horses board, each, plus feed, hay, and shavings.
I guess what irked me off was after you said you keep your horses there free, you complained about how often they go out to eat. Maybe they think that since you live with your boyfriends parents and dont have a mortgage you should be able to afford board on your horses, I couldnt tell you for sure. I myself would eat the cost of the hay they used, because board anywhere else is going to cost you a whole lot more.
As far as hay replacements, I like TNT chops. Its like alfalfa cubes, that arent compressed into cubes. I have fed it before as a sole hay source with no issues. Its not ideal, but in a pinch, its worked. I can dump a bag in a nice sized tupperware and it fits in the car. I do it at shows often. TSC also sells the compressed bales of hay, in my area they are $18 a bale. I think the extra money you pay for any hay substitute is going to cost more than the amount of hay they are using over their allotted share.