No, 75 gallons is not big enough. The footprint of a 75 is 4 feet by 18 inches, if I remember correctly. BGKs get to be 20 inches long, I think? Fully grown, he wouldn't even be able to turn around properly. You can ask about their requirements in the oddballs section.
Young clown loaches would be OK in a 75 gallon tank, and they are relatively slow growers. Eventually you would need to upgrade to a 6 foot tank for them to be comfortable. They are very active, and enjoy large groups of other loaches. I would not reccommend getting just two.
Bala sharks grow to about 13" I think. 75 gallons seems pretty tight for them, too.
Ideally this is not a good mix, even with a much larger tank. Each type would be happiest if you chose other tankmates. Maybe pick your favorite and build a tank on what would be ideal for them. Or if you go with a 75 gallon, pick something that would be comfortable in there. Like I said, though, if you do plan on upgrading your tank size later, I know the clowns at least would be good in there until the largest reached about 5 or 6 inches. Max size on clowns is something like 12 or 14 but not all reach that size.
