not much. thats a decent sized fish for a 75, he could hit about a foot. what filtration do you have? if you have a ton of filtration and do big frequent water changes (50% at least weekly) than you can probably add something. a small school of a smaller species of silver dollar (spotted silver dollars get to be about 4" tops) should be okay.
I dont know the compatibilty, water requirements etc. but i think some exadon paradoxis (spelling???) would look pretty cool. They are in the same family as tetras but have teeth and are pretty agressive and probably wont hurt your texas if it is big enough. They tend to school and make an attractive addition. I have never had them but my friend had some in a tank with some misc. fish and he had some others in with his pirranahs, a few would get knocked off but they are cheap and a good source of protein. Good luck.
Get a school of Giant Danios. They are VERY fast and NEVER stop swimming. You may lose one every blue moon but for the most part, they are perfect Dither fish.
Giant danios will grow to 6 inches (probably too big for a texas cichlid to eat) and there schooling fish, so you need atleast 6 of them.
So your saying a 75g with 1 12" texas cichlid will be able to handle 6 more 6 inch fish? I highly doubt that. I also doubt the giant danios will get eaten, unless you get buy a 4-5 inch texas cichlid and 1 inch giant danios, just have common sense and nobody will snack on tankmates.